diff --git a/pp.c b/pp.c
index 98edc8f37b92..65d0287e35cf 100644
--- a/pp.c
+++ b/pp.c
@@ -3434,12 +3434,15 @@ PP(pp_substr)
             SV* repl_sv_copy = NULL;
 
             if (repl_need_utf8_upgrade) {
-                repl_sv_copy = newSVsv(repl_sv);
+                repl_sv_copy = newSVpvn(repl, repl_len);
                 sv_utf8_upgrade(repl_sv_copy);
                 repl = SvPV_const(repl_sv_copy, repl_len);
             }
-            if (!SvOK(sv))
-                SvPVCLEAR(sv);
+
+            /* The earlier SvPV_force_nomg(sv, curlen) should have ensured
+             * that sv is SvOK, even if it wasn't beforehand. */
+            assert(SvOK(sv));
+
             sv_insert_flags(sv, byte_pos, byte_len, repl, repl_len, 0);
             SvREFCNT_dec(repl_sv_copy);
         }