diff --git a/src/string.c b/src/string.c
index 9918b2903..88575c7ca 100644
--- a/src/string.c
+++ b/src/string.c
@@ -1200,7 +1200,16 @@ mrb_str_cmp(mrb_state *mrb, mrb_value str1, mrb_value str2)
   mrb_int len1 = RSTR_LEN(s1);
   mrb_int len2 = RSTR_LEN(s2);
   mrb_int len = lesser(len1, len2);
-  mrb_int retval = memcmp(RSTR_PTR(s1), RSTR_PTR(s2), len);
+  mrb_int retval;
+
+  /* avoid UB: memcmp requires non-NULL pointers even when size is 0 */
+  if (len == 0) {
+    retval = 0;
+  }
+  else {
+    retval = memcmp(RSTR_PTR(s1), RSTR_PTR(s2), len);
+  }
+
   if (retval == 0) {
     if (len1 == len2) return 0;
     if (len1 > len2)  return 1;