diff --git a/sv.c b/sv.c
index 5ba3b1b62cea..6d8dd2683ae0 100644
--- a/sv.c
+++ b/sv.c
@@ -4912,10 +4912,11 @@ Perl_sv_setsv_cow(pTHX_ SV **pdsv, SV *ssv)
                     sv_dump(dsv);
     }
 #endif
-    if (!CHECK_COWBUF_THRESHOLD(cur, len)
-        || ! CHECK_COW_THRESHOLD(cur, len)) {
+    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 %zu/%zu not appropriate to COW\n", cur, len));
+                              "Fast copy on write: Sizes %zu/%zu %p/%p not appropriate to COW\n", cur, len, dsv ? (void *)SvPVX(dsv) : (void *)0, (void*)SvPVX(ssv)));
         return FALSE;
     }
     if (dsv) {