diff --git a/debug/debugFlags.c b/debug/debugFlags.c
index 12e00f32..9a71b392 100644
--- a/debug/debugFlags.c
+++ b/debug/debugFlags.c
@@ -208,7 +208,7 @@ DebugSet(clientID, argc, argv, value)
 
     if (id < 0 || id >= debugNumClients)
     {
-	TxError("DebugSet: bad client id %d\n", clientID);
+	TxError("DebugSet: bad client id %lu\n", (unsigned long)clientID);
 	return;
     }
     dc = &debugClients[id];