diff --git a/csq.c b/csq.c
index 49812d4de..a7d5d5883 100644
--- a/csq.c
+++ b/csq.c
@@ -3374,7 +3374,7 @@ void vbuf_flush(args_t *args, uint32_t pos)
         i = rbuf_shift(&args->vcf_rbuf);
         assert( i>=0 );
         vbuf = args->vcf_buf[i];
-        int pos = vbuf->n ? vbuf->vrec[0]->line->pos : -1;
+        int pos = vbuf->n ? vbuf->vrec[0]->line->pos : CSI_COOR_EMPTY;
         for (i=0; i<vbuf->n; i++)
         {
             vrec_t *vrec = vbuf->vrec[i];
@@ -3413,7 +3413,7 @@ void vbuf_flush(args_t *args, uint32_t pos)
             bcf_empty(vrec->line);
             vrec->line->pos = save_pos;
         }
-        if ( pos!=-1 )
+        if ( pos!=CSI_COOR_EMPTY )
         {
             khint_t k = kh_get(pos2vbuf, args->pos2vbuf, pos);
             if ( k != kh_end(args->pos2vbuf) ) kh_del(pos2vbuf, args->pos2vbuf, k);
@@ -4169,7 +4169,7 @@ static void process(args_t *args, bcf1_t **rec_ptr)
     }
 
     bcf1_t *rec = *rec_ptr;
-    static int32_t prev_rid = -1, prev_pos = -1;
+    static int32_t prev_rid = -1, prev_pos = CSI_COOR_EMPTY;
     if ( prev_rid!=rec->rid )
     {
         prev_rid = rec->rid;
diff --git a/test/telomere.0.out b/test/telomere.0.out
new file mode 100644
index 000000000..8b88f8412
--- /dev/null
+++ b/test/telomere.0.out
@@ -0,0 +1 @@
+22	0	id0	C	G	.	.	.
diff --git a/test/telomere.1.out b/test/telomere.1.out
new file mode 100644
index 000000000..ca28f4d72
--- /dev/null
+++ b/test/telomere.1.out
@@ -0,0 +1 @@
+22	1	id1	C	G	.	.	.
diff --git a/test/telomere.vcf b/test/telomere.vcf
new file mode 100644
index 000000000..243cab79c
--- /dev/null
+++ b/test/telomere.vcf
@@ -0,0 +1,5 @@
+##fileformat=VCFv4.2
+##contig=<ID=22>
+#CHROM	POS	ID	REF	ALT	QUAL	FILTER	INFO
+22	0	id0	C	G	.	.	.
+22	1	id1	C	G	.	.	.
diff --git a/test/test.pl b/test/test.pl
index 0d8490240..713704e7a 100755
--- a/test/test.pl
+++ b/test/test.pl
@@ -38,6 +38,8 @@
 run_test(\&test_tabix,$opts,in=>'merge.a',reg=>'1:3000151-3000151',out=>'tabix.1.3000151.out');
 run_test(\&test_index,$opts,in=>'large_chrom_csi_limit',reg=>'chr20:1-2147483647',out=>'large_chrom_csi_limit.20.1.2147483647.out'); # 2147483647 (1<<31-1) is the current chrom limit for csi. bcf conversion and indexing fail above this
 run_test(\&test_index,$opts,in=>'large_chrom_csi_limit',reg=>'chr20',out=>'large_chrom.20.1.2147483647.out'); # this fails until bug resolved
+run_test(\&test_index,$opts,in=>'telomere',reg=>'22:0',out=>'telomere.0.out');
+run_test(\&test_index,$opts,in=>'telomere',reg=>'22:1',out=>'telomere.1.out');
 run_test(\&test_vcf_idxstats,$opts,in=>'idx',args=>'-s',out=>'idx.out');
 run_test(\&test_vcf_idxstats,$opts,in=>'idx',args=>'-n',out=>'idx_count.out');
 run_test(\&test_vcf_idxstats,$opts,in=>'empty',args=>'-s',out=>'empty.idx.out');
diff --git a/vcfconcat.c b/vcfconcat.c
index 74fd036b8..48ffff337 100644
--- a/vcfconcat.c
+++ b/vcfconcat.c
@@ -1,6 +1,6 @@
 /*  vcfconcat.c -- Concatenate or combine VCF/BCF files.
 
-    Copyright (C) 2013-2021 Genome Research Ltd.
+    Copyright (C) 2013-2023 Genome Research Ltd.
 
     Author: Petr Danecek <pd3@sanger.ac.uk>
 
@@ -39,6 +39,8 @@ THE SOFTWARE.  */
 #include <sys/time.h>
 #include "bcftools.h"
 
+#define EMPTY_FILE -3
+
 typedef struct _args_t
 {
     bcf_srs_t *files;
@@ -95,11 +97,11 @@ static void init_data(args_t *args)
         if ( args->phased_concat )
         {
             int ret = bcf_read(fp, hdr, line);
-            if ( ret!=0 ) args->start_pos[i] = -2;  // empty file
+            if ( ret!=0 ) args->start_pos[i] = EMPTY_FILE;
             else
             {
                 int chrid = bcf_hdr_id2int(args->out_hdr,BCF_DT_CTG,bcf_seqname(hdr,line));
-                args->start_pos[i] = chrid==prev_chrid ? line->pos : -1;
+                args->start_pos[i] = chrid==prev_chrid ? line->pos : CSI_COOR_EMPTY;
                 prev_chrid = chrid;
             }
         }
@@ -171,11 +173,11 @@ static void init_data(args_t *args)
         int nok = 0;
         while (1)
         {
-            while ( nok<args->nfnames && args->start_pos[nok]!=-2 ) nok++;
+            while ( nok<args->nfnames && args->start_pos[nok]!=EMPTY_FILE ) nok++;
             if ( nok==args->nfnames ) break;
 
             i = nok;
-            while ( i<args->nfnames && args->start_pos[i]==-2 ) i++;
+            while ( i<args->nfnames && args->start_pos[i]==EMPTY_FILE ) i++;
             if ( i==args->nfnames ) break;
 
             int tmp = args->start_pos[nok]; args->start_pos[nok] = args->start_pos[i]; args->start_pos[i] = tmp;
@@ -185,7 +187,7 @@ static void init_data(args_t *args)
         args->nfnames = nok;
 
         for (i=1; i<args->nfnames; i++)
-            if ( args->start_pos[i-1]!=-1 && args->start_pos[i]!=-1 && args->start_pos[i]<args->start_pos[i-1] )
+            if ( args->start_pos[i-1]!=CSI_COOR_EMPTY && args->start_pos[i]!=CSI_COOR_EMPTY && args->start_pos[i]<args->start_pos[i-1] )
                 error("The files not in ascending order: %d in %s, %d in %s\n", args->start_pos[i-1]+1,args->fnames[i-1],args->start_pos[i]+1,args->fnames[i]);
 
         args->prev_chr = -1;
@@ -264,7 +266,7 @@ static void phased_flush(args_t *args)
         bcf1_t *brec = args->buf[i+1];
 
         int nGTs = bcf_get_genotypes(ahdr, arec, &args->GTa, &args->mGTa);
-        if ( nGTs < 0 ) 
+        if ( nGTs < 0 )
         {
             if ( !gt_absent_warned )
             {
@@ -359,7 +361,7 @@ static void phased_flush(args_t *args)
             bcf_update_format_int32(args->out_hdr,rec,"PQ",args->phase_qual,nsmpl);
             PQ_printed = 1;
             for (j=0; j<nsmpl; j++)
-                if ( args->phase_qual[j] < args->min_PQ ) 
+                if ( args->phase_qual[j] < args->min_PQ )
                 {
                     args->phase_set[j] = rec->pos+1;
                     args->phase_set_changed = 1;
@@ -404,7 +406,7 @@ static void phased_push(args_t *args, bcf1_t *arec, bcf1_t *brec, int is_overlap
         if ( args->seen_seq[chr_id] ) error("The chromosome block %s is not contiguous\n", arec ? bcf_seqname(ahdr,arec) : bcf_seqname(bhdr,brec));
         args->seen_seq[chr_id] = 1;
         args->prev_chr = chr_id;
-        args->prev_pos_check = -1;
+        args->prev_pos_check = CSI_COOR_EMPTY;
     }
 
     if ( !is_overlap )
@@ -463,12 +465,12 @@ static void concat(args_t *args)
                 new_file = 1;
 
                 args->ifname++;
-                if ( args->start_pos[args->ifname-1]==-1 ) break;   // new chromosome, start with only one file open
-                if ( args->ifname < args->nfnames && args->start_pos[args->ifname]==-1 ) break; // next file starts on a different chromosome
+                if ( args->start_pos[args->ifname-1]==CSI_COOR_EMPTY ) break;   // new chromosome, start with only one file open
+                if ( args->ifname < args->nfnames && args->start_pos[args->ifname]==CSI_COOR_EMPTY ) break; // next file starts on a different chromosome
             }
 
             // is there a line from the previous run? Seek the newly opened reader to that position
-            int seek_pos = -1;
+            int seek_pos = CSI_COOR_EMPTY;
             int seek_chr = -1;
             if ( bcf_sr_has_line(args->files,0) )
             {
@@ -521,11 +523,12 @@ static void concat(args_t *args)
 
                 // This can happen after bcf_sr_seek: indel may start before the coordinate which we seek to.
                 if ( seek_chr>=0 && seek_pos>line->pos && seek_chr==bcf_hdr_name2id(args->out_hdr, bcf_seqname(args->files->readers[ir].header,line)) ) continue;
-                seek_pos = seek_chr = -1;
+                seek_pos = CSI_COOR_EMPTY;
+                seek_chr = -1;
 
                 //  Check if the position overlaps with the next, yet unopened, reader
                 int must_seek = 0;
-                while ( args->ifname < args->nfnames && args->start_pos[args->ifname]!=-1 && line->pos >= args->start_pos[args->ifname] )
+                while ( args->ifname < args->nfnames && args->start_pos[args->ifname]!=CSI_COOR_EMPTY && line->pos >= args->start_pos[args->ifname] )
                 {
                     must_seek = 1;
                     if ( !bcf_sr_add_reader(args->files,args->fnames[args->ifname]) ) error("Failed to open %s: %s\n", args->fnames[args->ifname],bcf_sr_strerror(args->files->errnum));
@@ -618,7 +621,7 @@ static void concat(args_t *args)
                     if ( chr_id<0 ) error("\nThe sequence \"%s\" not defined in the header: %s\n(Quick workaround: index the file.)\n", tmp.s, args->fnames[i]);
                     if ( prev_chr_id!=chr_id )
                     {
-                        prev_pos = -1;
+                        prev_pos = CSI_COOR_EMPTY;
                         if ( args->seen_seq[chr_id] )
                             error("\nThe chromosome block %s is not contiguous, consider running with -a.\n", tmp.s);
                     }
@@ -643,7 +646,7 @@ static void concat(args_t *args)
 
                     if ( prev_chr_id!=line->rid )
                     {
-                        prev_pos = -1;
+                        prev_pos = CSI_COOR_EMPTY;
                         if ( args->seen_seq[line->rid] )
                             error("\nThe chromosome block %s is not contiguous, consider running with -a.\n", bcf_seqname(args->out_hdr, line));
                     }
@@ -980,7 +983,7 @@ int main_vcfconcat(int argc, char *argv[])
             case 'R': args->regions_list = optarg; args->regions_is_file = 1; break;
             case 'd': args->remove_dups = optarg; break;
             case 'D': args->remove_dups = "exact"; break;
-            case 'q': 
+            case 'q':
                 args->min_PQ = strtol(optarg,&tmp,10);
                 if ( *tmp ) error("Could not parse argument: --min-PQ %s\n", optarg);
                 break;
diff --git a/vcfconvert.c b/vcfconvert.c
index ce5ed9981..dac134d05 100644
--- a/vcfconvert.c
+++ b/vcfconvert.c
@@ -1,6 +1,6 @@
 /*  vcfconvert.c -- convert between VCF/BCF and related formats.
 
-    Copyright (C) 2013-2021 Genome Research Ltd.
+    Copyright (C) 2013-2023 Genome Research Ltd.
 
     Author: Petr Danecek <pd3@sanger.ac.uk>
 
@@ -59,7 +59,7 @@ struct _args_t
     bcf_hdr_t *header;
     void (*convert_func)(struct _args_t *);
     struct {
-        int total, skipped, hom_rr, het_ra, hom_aa, het_aa, missing; 
+        int total, skipped, hom_rr, het_ra, hom_aa, het_aa, missing;
     } n;
     kstring_t str;
     int32_t *gts;
@@ -160,7 +160,7 @@ static int _set_chrom_pos_ref_alt(tsv_t *tsv, bcf1_t *rec, void *usr)
     // REF,ALT
     args->str.l = 0;
     se = ++ss;
-    while ( se < tsv->se && *se!='_' ) se++; 
+    while ( se < tsv->se && *se!='_' ) se++;
     if ( *se!='_' ) return -1;
     kputsn(ss,se-ss,&args->str);
     ss = ++se;
@@ -202,14 +202,14 @@ static int tsv_setter_chrom_pos_ref_alt_or_id(tsv_t *tsv, bcf1_t *rec, void *usr
 {
     args_t *args = (args_t*)usr;
     if ( _set_chrom_pos_ref_alt(tsv,rec,usr)==0 )  return 0;
-    rec->pos = -1;  // mark the record as unset
+    rec->pos = CSI_COOR_EMPTY;  // mark the record as unset
     if ( !args->output_vcf_ids) return 0;
     return tsv_setter_id(tsv,rec,usr);
 }
 static int tsv_setter_chrom_pos_ref_alt_id_or_die(tsv_t *tsv, bcf1_t *rec, void *usr)
 {
     args_t *args = (args_t*)usr;
-    if ( rec->pos!=-1 )
+    if ( rec->pos!=CSI_COOR_EMPTY )
     {
         if ( !args->output_vcf_ids ) return 0;
         return tsv_setter_id(tsv,rec,usr);
@@ -269,12 +269,12 @@ static int tsv_setter_gt_gp(tsv_t *tsv, bcf1_t *rec, void *usr)
         if ( aa >= ab )
         {
             if ( aa >= bb ) args->gts[2*i+0] = args->gts[2*i+1] = bcf_gt_unphased(0);
-            else args->gts[2*i+0] = args->gts[2*i+1] = bcf_gt_unphased(1); 
+            else args->gts[2*i+0] = args->gts[2*i+1] = bcf_gt_unphased(1);
         }
-        else if ( ab >= bb ) 
+        else if ( ab >= bb )
         {
             args->gts[2*i+0] = bcf_gt_unphased(0);
-            args->gts[2*i+1] = bcf_gt_unphased(1); 
+            args->gts[2*i+1] = bcf_gt_unphased(1);
         }
         else args->gts[2*i+0] = args->gts[2*i+1] = bcf_gt_unphased(1);
     }
@@ -293,7 +293,7 @@ static int tsv_setter_haps(tsv_t *tsv, bcf1_t *rec, void *usr)
     else { a0 = bcf_gt_phased(0); a1 = bcf_gt_phased(1); }
 
     // up is short for "unphased"
-    int nup = 0; 
+    int nup = 0;
     for (i=0; i<nsamples; i++)
     {
         char *ss = tsv->ss + 4*i + nup;
@@ -324,11 +324,11 @@ static int tsv_setter_haps(tsv_t *tsv, bcf1_t *rec, void *usr)
                 break;
             default :
                 fprintf(stderr,"Could not parse: [%c][%s]\n", ss[all*2+up],tsv->ss);
-                return -1; 
+                return -1;
             }
             if( ss[all*2+up+1]=='*' ) up = up + 1;
         }
-        
+
         if(up && up != 2)
         {
             fprintf(stderr,"Missing unphased marker '*': [%c][%s]", ss[2+up], tsv->ss);
@@ -356,13 +356,13 @@ static int tsv_setter_haps(tsv_t *tsv, bcf1_t *rec, void *usr)
 static void gensample_to_vcf(args_t *args)
 {
     /*
-     *  Inpute: IMPUTE2 output (indentation changed here for clarity): 
+     *  Inpute: IMPUTE2 output (indentation changed here for clarity):
      *
      *      20:62116619_C_T 20:62116619     62116619 C T 0.969 0.031 0 ...
      *      ---             20:62116698_C_A 62116698 C A 1     0     0 ...
      *
      *  Second column is expected in the form of CHROM:POS_REF_ALT. We use second
-     *  column because the first can be empty ("--") when filling sites from reference 
+     *  column because the first can be empty ("--") when filling sites from reference
      *  panel. When the option --vcf-ids is given, the first column is used to set the
      *  VCF ID.
      *
@@ -784,7 +784,7 @@ char *init_sample2sex(bcf_hdr_t *hdr, char *sex_fname)
     }
     for (i=0; i<nlines; i++) free(lines[i]);
     free(lines);
-    for (i=0; i<bcf_hdr_nsamples(hdr); i++) 
+    for (i=0; i<bcf_hdr_nsamples(hdr); i++)
         if ( !sample2sex[i] ) error("Missing sex for sample %s in %s\n", bcf_hdr_int2id(hdr, BCF_DT_SAMPLE, i),sex_fname);
     return sample2sex;
 }
@@ -847,7 +847,7 @@ static void vcf_to_gensample(args_t *args)
     if (sample_fname) fprintf(stderr, "Sample file: %s\n", sample_fname);
 
     // write samples file
-    if (sample_fname) 
+    if (sample_fname)
     {
         char *sample2sex = NULL;
         if ( args->sex_fname ) sample2sex = init_sample2sex(args->header,args->sex_fname);
@@ -877,7 +877,7 @@ static void vcf_to_gensample(args_t *args)
         return;
     }
 
-    int prev_rid = -1, prev_pos = -1;
+    int prev_rid = -1, prev_pos = CSI_COOR_EMPTY;
     int no_alt = 0, non_biallelic = 0, filtered = 0, ndup = 0, nok = 0;
     BGZF *gout = bgzf_open(gen_fname, gen_compressed ? "wg" : "wu");
     while ( bcf_sr_next_line(args->files) )
@@ -915,7 +915,7 @@ static void vcf_to_gensample(args_t *args)
             nok++;
         }
     }
-    fprintf(stderr, "%d records written, %d skipped: %d/%d/%d/%d no-ALT/non-biallelic/filtered/duplicated\n", 
+    fprintf(stderr, "%d records written, %d skipped: %d/%d/%d/%d no-ALT/non-biallelic/filtered/duplicated\n",
         nok, no_alt+non_biallelic+filtered+ndup, no_alt, non_biallelic, filtered, ndup);
 
     if ( str.m ) free(str.s);
@@ -976,7 +976,7 @@ static void vcf_to_haplegendsample(args_t *args)
     {
         char *sample2sex = NULL;
         if ( args->sex_fname ) sample2sex = init_sample2sex(args->header,args->sex_fname);
-        
+
         int i;
         BGZF *sout = bgzf_open(sample_fname, sample_compressed ? "wg" : "wu");
         str.l = 0;
@@ -1078,7 +1078,7 @@ static void vcf_to_hapsample(args_t *args)
         kputs("%CHROM:%POS\\_%REF\\_%FIRST_ALT %ID %POS %REF %FIRST_ALT ", &str);
     else
         kputs("%CHROM %CHROM:%POS\\_%REF\\_%FIRST_ALT %POS %REF %FIRST_ALT ", &str);
-    
+
     if ( args->hap2dip )
         kputs("%_GT_TO_HAP2\n", &str);
     else
@@ -1229,7 +1229,7 @@ static inline int tsv_setter_aa1(args_t *args, char *ss, char *se, int alleles[]
     if ( alleles[a0]<0 ) alleles[a0] = (*nals)++;
     if ( alleles[a1]<0 ) alleles[a1] = (*nals)++;
 
-    gts[0] = bcf_gt_unphased(alleles[a0]); 
+    gts[0] = bcf_gt_unphased(alleles[a0]);
     gts[1] = ss[1] ? bcf_gt_unphased(alleles[a1]) : bcf_int32_vector_end;
 
     if ( ref==a0 && ref==a1  ) args->n.hom_rr++;    // hom ref: RR
@@ -1265,7 +1265,7 @@ static int tsv_setter_aa(tsv_t *tsv, bcf1_t *rec, void *usr)
         }
         ret = tsv_setter_aa1(args, tsv->ss, tsv->se, alleles, &nals, iref, args->gts+i*2);
         if ( ret==-1 ) error("Error parsing the site %s:%"PRId64", expected two characters\n", bcf_hdr_id2name(args->header,rec->rid),(int64_t) rec->pos+1);
-        if ( ret==-2 ) 
+        if ( ret==-2 )
         {
             // something else than a SNP
             free(ref);
@@ -1275,7 +1275,7 @@ static int tsv_setter_aa(tsv_t *tsv, bcf1_t *rec, void *usr)
 
     args->str.l = 0;
     kputc(ref[0], &args->str);
-    for (i=0; i<5; i++) 
+    for (i=0; i<5; i++)
     {
         if ( alleles[i]>0 )
         {
@@ -1419,7 +1419,7 @@ static void gvcf_to_vcf(args_t *args)
         {
             int pass = filter_test(args->filter, line, NULL);
             if ( args->filter_logic & FLT_EXCLUDE ) pass = pass ? 0 : 1;
-            if ( !pass ) 
+            if ( !pass )
             {
                 if ( bcf_write(out_fh,hdr,line)!=0  ) error("[%s] Error: cannot write to %s\n", __func__,args->outfname);
                 continue;
@@ -1667,7 +1667,7 @@ int main_vcfconvert(int argc, char *argv[])
         else args->infname = argv[optind];
     }
     if ( !args->infname ) usage();
-    
+
     if ( args->convert_func ) args->convert_func(args);
     else vcf_to_vcf(args);
 
diff --git a/vcfmerge.c b/vcfmerge.c
index 621f4102c..def553188 100644
--- a/vcfmerge.c
+++ b/vcfmerge.c
@@ -1,6 +1,6 @@
 /*  vcfmerge.c -- Merge multiple VCF/BCF files to create one multi-sample file.
 
-    Copyright (C) 2012-2022 Genome Research Ltd.
+    Copyright (C) 2012-2023 Genome Research Ltd.
 
     Author: Petr Danecek <pd3@sanger.ac.uk>
 
@@ -828,7 +828,7 @@ void maux_reset(maux_t *ma, int *rid_tab)
     }
     const char *chr = NULL;
     ma->nals  = 0;
-    ma->pos   = -1;
+    ma->pos   = CSI_COOR_EMPTY;
     for (i=0; i<ma->n; i++)
     {
         if ( !bcf_sr_has_line(ma->files,i) ) continue;
@@ -897,7 +897,7 @@ void merge_chrom2qual(args_t *args, bcf1_t *out)
     bcf_float_set_missing(out->qual);
 
     // CHROM, POS, ID, QUAL
-    out->pos = -1;
+    out->pos = CSI_COOR_EMPTY;
     for (i=0; i<files->nreaders; i++)
     {
         bcf1_t *line = maux_get_line(args, i);
@@ -916,7 +916,7 @@ void merge_chrom2qual(args_t *args, bcf1_t *out)
         }
 
         // position
-        if ( out->pos==-1 )
+        if ( out->pos==CSI_COOR_EMPTY )
         {
             const char *chr = hdr->id[BCF_DT_CTG][line->rid].key;
             out->rid = bcf_hdr_name2id(out_hdr, chr);
diff --git a/vcfnorm.c b/vcfnorm.c
index 9538f8d01..213edd501 100644
--- a/vcfnorm.c
+++ b/vcfnorm.c
@@ -1,6 +1,6 @@
 /*  vcfnorm.c -- Left-align and normalize indels.
 
-    Copyright (C) 2013-2022 Genome Research Ltd.
+    Copyright (C) 2013-2023 Genome Research Ltd.
 
     Author: Petr Danecek <pd3@sanger.ac.uk>
 
@@ -1819,7 +1819,7 @@ static void flush_buffer(args_t *args, htsFile *file, int n)
 {
     bcf1_t *line;
     int i, k;
-    int prev_rid = -1, prev_pos = -1, prev_type = 0;
+    int prev_rid = -1, prev_pos = CSI_COOR_EMPTY, prev_type = 0;
     for (i=0; i<n; i++)
     {
         k = rbuf_shift(&args->rbuf);
@@ -2020,7 +2020,7 @@ static void normalize_vcf(args_t *args)
     if ( bcf_hdr_write(args->out, args->out_hdr)!=0 ) error("[%s] Error: cannot write to %s\n", __func__,args->output_fname);
 
     bcf1_t *line;
-    int prev_rid = -1, prev_pos = -1, prev_type = 0;
+    int prev_rid = -1, prev_pos = CSI_COOR_EMPTY, prev_type = 0;
     while ( (line = next_atomized_line(args)) )
     {
         args->ntotal++;