diff --git a/lib/qmp_client/messages.rb b/lib/qmp_client/messages.rb
index ec59e21..2d7279f 100644
--- a/lib/qmp_client/messages.rb
+++ b/lib/qmp_client/messages.rb
@@ -17,7 +17,7 @@ module QMPClient
     # Raised when we're trying to convert raw text into a message, but failing
     class ParserError < StandardError
       attr_reader :upstream_error
-      def initialize(message, upstream)
+      def initialize(message, upstream = nil)
         super(message)
         @upstream_error = upstream
       end