diff --git a/include/__locale b/include/__locale
index 67f191c0..bf0d0e1b 100644
--- a/include/__locale
+++ b/include/__locale
@@ -451,7 +451,7 @@ public:
     _LIBCPP_ALWAYS_INLINE
     bool is(mask __m, char_type __c) const
     {
-        return isascii(__c) ? __tab_[__c] & __m : false;
+        return isascii(__c) ? (__tab_ ? __tab_[__c] & __m : isctype(__c, __m)) : false;
     }
 
     _LIBCPP_ALWAYS_INLINE