diff --git a/src/hlua.c b/src/hlua.c
index 4c5598d..d8e538b 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -838,7 +838,7 @@
 			*(p-1) = '.';
 			break;
 		}
-		if (isprint(*msg))
+		if (isprint((unsigned char)*msg))
 			*p = *msg;
 		else
 			*p = '.';