1 files changed, 2 insertions, 2 deletions
diff --git a/Xtrans.c b/Xtrans.c
index d9e32d0..11f6161 100644
--- a/Xtrans.c
+++ b/Xtrans.c
@@ -166,8 +166,8 @@ TRANS(SelectTransport) (const char *protocol)
     protobuf[PROTOBUFSIZE-1] = '\0';
 
     for (i = 0; i < PROTOBUFSIZE && protobuf[i] != '\0'; i++)
-	if (isupper (protobuf[i]))
-	    protobuf[i] = tolower (protobuf[i]);
+	if (isupper ((unsigned char)protobuf[i]))
+	    protobuf[i] = tolower ((unsigned char)protobuf[i]);
 
     /* Look at all of the configured protocols */