diff --git a/exprtk.hpp b/exprtk.hpp
index 4a4db54..a45c330 100644
--- a/exprtk.hpp
+++ b/exprtk.hpp
@@ -129,7 +129,8 @@ namespace exprtk
       inline bool is_letter(const char_t c)
       {
          return (('a' <= c) && (c <= 'z')) ||
-                (('A' <= c) && (c <= 'Z')) ;
+                (('A' <= c) && (c <= 'Z')) ||
+                ((c < 0) || (127 < c)) ;
       }
 
       inline bool is_digit(const char_t c)