diff --git a/main.ml b/main.ml
index 595158a..976535a 100644
--- a/main.ml
+++ b/main.ml
@@ -2402,6 +2402,12 @@ let rec generate (letfuncs, strings, typedefs, exps) =
     appfmt buf "mov rax, %d" @@ tagged_int 0 ;
     appstr buf "ret" ;
     appstr buf "" ;
+    appstr buf "aqaml_string_create:" ;
+    appstr buf @@ untag_int "rax" ;
+    appstr buf "mov rdi, rax" ;
+    appstr buf "call aqaml_string_create_detail@PLT" ;
+    appstr buf "ret" ;
+    appstr buf "" ;
     appstr buf "aqaml_string_blit:" ;
     appstr buf @@ untag_int "rbx" ;
     appstr buf @@ untag_int "rsi" ;
diff --git a/stdlib.ml b/stdlib.ml
index 33ed351..726e751 100644
--- a/stdlib.ml
+++ b/stdlib.ml
@@ -19,6 +19,8 @@ module Bytes = struct
 
   external set : bytes -> int -> char -> unit = "aqaml_string_set"
 
+  external create : int -> bytes = "aqaml_string_create"
+
   external blit :
     bytes -> int -> bytes -> int -> int -> unit
     = "aqaml_string_blit"
@@ -39,6 +41,8 @@ module String = struct
 
   external set : string -> int -> char -> unit = "aqaml_string_set"
 
+  external create : int -> bytes = "aqaml_string_create"
+
   external blit :
     string -> int -> bytes -> int -> int -> unit
     = "aqaml_string_blit"
diff --git a/test.ml b/test.ml
index f0ab783..c8924e3 100644
--- a/test.ml
+++ b/test.ml
@@ -1517,3 +1517,11 @@ let dst = Bytes.of_string "def  " in
 String.blit src 1 dst 2 3 ;
 test src "abcd" ;
 test dst @@ Bytes.of_string "debcd"
+
+;;
+let src = "abcd" in
+let dst = Bytes.create 5 in
+dst.[0] <- 'd' ;
+dst.[1] <- 'e' ;
+String.blit src 1 dst 2 3 ;
+test dst @@ Bytes.of_string "debcd"
diff --git a/utility.c b/utility.c
index 063129b..6e03ffb 100644
--- a/utility.c
+++ b/utility.c
@@ -29,6 +29,7 @@ typedef struct AQamlValue {
 } AQamlValue;
 
 uint64_t aqaml_string_length_detail(uint64_t ptr);
+uint64_t aqaml_string_create_detail(uint64_t len);
 
 void *aqaml_malloc_detail(uint32_t size)
 {
@@ -103,17 +104,13 @@ uint64_t aqaml_concat_string_detail(uint64_t lhs_src, uint64_t rhs_src)
     AQamlValue lhs = get_value(lhs_src), rhs = get_value(rhs_src);
     // assert(lhs.kind == AQAML_STRING && rhs.kind == AQAML_STRING);
 
-    uint64_t ret_src = aqaml_alloc_block((lhs_len + rhs_len) / 8 + 1, 0, 252);
+    uint64_t ret_src = aqaml_string_create_detail(lhs_len + rhs_len);
     AQamlValue ret = get_value(ret_src);
-    uint64_t space = 7 - (lhs_len + rhs_len) % 8;
 
     for (uint64_t i = 0; i < lhs_len; i++)
         ret.string->str[i] = lhs.string->str[i];
     for (uint64_t i = 0; i < rhs_len; i++)
         ret.string->str[i + lhs_len] = rhs.string->str[i];
-    for (uint64_t i = 0; i < space; i++)
-        ret.string->str[i + lhs_len + rhs_len] = 0u;
-    ret.string->str[lhs_len + rhs_len + space] = space;
 
     return ret_src;
 }
@@ -145,6 +142,16 @@ void aqaml_string_set_detail(uint64_t str_src, uint64_t index, uint64_t chr)
     val.string->str[index] = (uint8_t)chr;
 }
 
+uint64_t aqaml_string_create_detail(uint64_t len)
+{
+    uint64_t ret_src = aqaml_alloc_block(len / 8 + 1, 0, 252);
+    uint64_t space = 7 - len % 8;
+    AQamlValue ret = get_value(ret_src);
+    for (uint64_t i = 0; i < space; i++) ret.string->str[len + i] = 0;
+    ret.string->str[len + space] = space;
+    return ret_src;
+}
+
 void aqaml_string_blit_detail(uint64_t src_src, uint64_t srcoff,
                               uint64_t dst_src, uint64_t dstoff, uint64_t len)
 {