diff --git a/mrbgems/mruby-regexp/src/re_compile.c b/mrbgems/mruby-regexp/src/re_compile.c
index b43daf3df..3732f8255 100644
--- a/mrbgems/mruby-regexp/src/re_compile.c
+++ b/mrbgems/mruby-regexp/src/re_compile.c
@@ -766,6 +766,7 @@ first_set_walk(const re_inst *code, uint32_t code_len,
       pc = code[pc].offset;
       continue;
     case RE_CHAR:
+      if (code[pc].a >= 128) return FALSE;  /* non-ASCII: bm covers ASCII only */
       bm[code[pc].a >> 3] |= (1 << (code[pc].a & 7));
       return TRUE;
     case RE_CLASS: {