diff --git a/include/ac_int.h b/include/ac_int.h
index 7672893..76bbfc0 100644
--- a/include/ac_int.h
+++ b/include/ac_int.h
@@ -2484,7 +2484,7 @@ __AC_INT_UTILITY_BASE
     // Zero Pads if str is too short, throws ms bits away if str is too long
     // Asserts if anything other than 0-9a-fA-F is encountered
     ac_int<W,S> res = 0;
-    while(str) {
+    while(*str) {
       char c = *str;
       int h = 0;
       if(c >= '0' && c <= '9')