diff --git a/include/mp/proxy-types.h b/include/mp/proxy-types.h
index f11fb359..8236b0c9 100644
--- a/include/mp/proxy-types.h
+++ b/include/mp/proxy-types.h
@@ -881,7 +881,7 @@ ::capnp::Void BuildPrimitive(InvokeContext& invoke_context, Value&&, TypeList<::
     return {};
 }
 
-inline static bool BuildPrimitive(InvokeContext& invoke_context, std::vector<bool>::reference value, TypeList<bool>)
+inline static bool BuildPrimitive(InvokeContext& invoke_context, std::vector<bool>::const_reference value, TypeList<bool>)
 {
     return value;
 }