diff --git a/names.c b/names.c
index d5182d88..00c79fee 100644
--- a/names.c
+++ b/names.c
@@ -97,7 +97,7 @@ parse_window_name(const char *in)
 
 	if (*name != '\0') {
 		ptr = name + strlen(name) - 1;
-		while (ptr > name && !isalnum(*ptr))
+		while (ptr > name && !isalnum((u_char)*ptr))
 			*ptr-- = '\0';
 	}