diff --git a/src/nix/doctor.cc b/src/nix/doctor.cc
index b608b9d59efa714758aa280fb647b6e6c3368998..1ce3efcb04152cd5831843b7fe965e26e7f8a7fa 100644
--- a/src/nix/doctor.cc
+++ b/src/nix/doctor.cc
@@ -91,7 +91,7 @@ struct CmdDoctor : StoreCommand
 
     void checkStoreProtocol(unsigned int storeProto)
     {
-        auto clientProto = GET_PROTOCOL_MAJOR(SERVE_PROTOCOL_VERSION) == GET_PROTOCOL_MAJOR(storeProto)
+        unsigned int clientProto = GET_PROTOCOL_MAJOR(SERVE_PROTOCOL_VERSION) == GET_PROTOCOL_MAJOR(storeProto)
             ? SERVE_PROTOCOL_VERSION
             : PROTOCOL_VERSION;