diff --git a/plugins/commands/serve/mappers/wrappers.rb b/plugins/commands/serve/mappers/wrappers.rb
index 544b87051..24c9fb45a 100644
--- a/plugins/commands/serve/mappers/wrappers.rb
+++ b/plugins/commands/serve/mappers/wrappers.rb
@@ -205,7 +205,7 @@ module VagrantPlugins
         end
 
         def converter(s)
-          Google::Protobuf::StringValue.new(value: s)
+          Google::Protobuf::StringValue.new(value: s.to_s)
         end
       end