diff --git a/pong.proto b/pong.proto
index 7606fa2..416513f 100644
--- a/pong.proto
+++ b/pong.proto
@@ -209,7 +209,7 @@ message Pong {
   // Unknown keys must be ignored by the client. Max 32 entries;
   // each key MUST NOT exceed 16 bytes. Clients MUST reject a Pong
   // carrying a gauges map that violates these bounds.
-  map<string, int64> gauges = 16;
+  map<string, uint64> gauges = 16;
 
   // Optional diagnostic line the Node wants surfaced to the user.
   // Usually empty. When set, formatted as "X WWW": a single