diff --git a/bacnet-stack/src/bacapp.c b/bacnet-stack/src/bacapp.c
index 9b6ce5ad..227e97e2 100644
--- a/bacnet-stack/src/bacapp.c
+++ b/bacnet-stack/src/bacapp.c
@@ -744,10 +744,11 @@ bool bacapp_print_value(
                     characterstring_value(&value->type.Character_String);
                 fprintf(stream, "\"");
                 for (i = 0; i < len; i++) {
-                    if (isprint(*char_str))
+                    if (isprint(*((unsigned char *)char_str))) {
                         fprintf(stream, "%c", *char_str);
-                    else
+                    } else {
                         fprintf(stream, ".");
+                    }
                     char_str++;
                 }
                 fprintf(stream, "\"");