diff --git a/src/lj_opt_fold.c b/src/lj_opt_fold.c
index e3fe8bbf..0c5a4a6a 100644
--- a/src/lj_opt_fold.c
+++ b/src/lj_opt_fold.c
@@ -605,7 +605,7 @@ LJFOLDF(bufput_bufstr)
       return ref;
     }
     /* Replay puts to global temporary buffer. */
-    if (IR(hdr)->op2 == IRBUFHDR_RESET) {
+    if (IR(hdr)->op2 == IRBUFHDR_RESET && !irt_isphi(fright->t)) {
       IRIns *ir = IR(fright->op1);
       /* For now only handle single string.reverse .lower .upper .rep. */
       if (ir->o == IR_CALLL &&