diff --git a/ext/-test-/string/coderange.c b/ext/-test-/string/coderange.c
index f6f58c4230..2f1e6a06a8 100644
--- a/ext/-test-/string/coderange.c
+++ b/ext/-test-/string/coderange.c
@@ -15,6 +15,7 @@ str_coderange(VALUE str)
       case ENC_CODERANGE_BROKEN:
 	return sym_broken;
     }
+    rb_bug("wrong condition of coderange");
     UNREACHABLE;
 }