diff --git a/src/nix/doctor.cc b/src/nix/doctor.cc
index b608b9d59..1ce3efcb0 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;