diff --git a/exprtk.hpp b/exprtk.hpp
index 4ce0848..69808c5 100644
--- a/exprtk.hpp
+++ b/exprtk.hpp
@@ -175,7 +175,7 @@ namespace exprtk
 
       inline bool is_valid_string_char(const char_t c)
       {
-         return std::isprint(static_cast<uchar_t>(c)) ||
+         return !std::iscntrl(static_cast<uchar_t>(c)) ||
                 is_whitespace(c);
       }