diff --git a/string.c b/string.c
index 3feeb8c705..5a91f6bcda 100644
--- a/string.c
+++ b/string.c
@@ -5087,7 +5087,7 @@ rb_str_sub_bang(int argc, VALUE *argv, VALUE str)
 	if (rlen != plen) {
 	    memmove(p + beg0 + rlen, p + beg0 + plen, len - beg0 - plen);
 	}
-	memcpy(p + beg0, rp, rlen);
+	memmove(p + beg0, rp, rlen);
 	len += rlen - plen;
 	STR_SET_LEN(str, len);
 	TERM_FILL(&RSTRING_PTR(str)[len], TERM_LEN(str));