diff --git a/regexec.c b/regexec.c
index 0c0d38b73797..965a42db3fed 100644
--- a/regexec.c
+++ b/regexec.c
@@ -3547,7 +3547,7 @@ S_reg_set_capture_string(pTHX_ REGEXP * const rx,
 #endif
         {
 #ifdef PERL_ANY_COW
-        didnt_cow:
+        didnt_cow: ;
 #endif
             SSize_t min = 0;
             SSize_t max = strend - strbeg;
diff --git a/sv.c b/sv.c
index ac5337a33b9c..735775704bd4 100644
--- a/sv.c
+++ b/sv.c
@@ -4912,6 +4912,13 @@ Perl_sv_setsv_cow(pTHX_ SV **pdsv, SV *ssv)
                     sv_dump(dsv);
     }
 #endif
+    if (!SvIsCOW(ssv) &&
+        (!CHECK_COWBUF_THRESHOLD(cur, len)
+         || ! CHECK_COW_THRESHOLD(cur, len))) {
+        DEBUG_C(PerlIO_printf(Perl_debug_log,
+                              "Fast copy on write: Sizes not appropriate to COW\n"));
+        return FALSE;
+    }
     if (dsv) {
         if (SvTHINKFIRST(dsv))
             sv_force_normal_flags(dsv, SV_COW_DROP_PV);