diff --git a/sv.c b/sv.c
index ac5337a33b9cd..25de20bbca614 100644
--- a/sv.c
+++ b/sv.c
@@ -4912,6 +4912,12 @@ Perl_sv_setsv_cow(pTHX_ SV **pdsv, SV *ssv)
                     sv_dump(dsv);
     }
 #endif
+    if (!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);