diff --git a/src/ejabberd_xmlrpc.erl b/src/ejabberd_xmlrpc.erl
index b1bd164a6..904604fc9 100644
--- a/src/ejabberd_xmlrpc.erl
+++ b/src/ejabberd_xmlrpc.erl
@@ -444,6 +444,8 @@ format_arg(Arg, binary) when is_list(Arg) -> list_to_binary(Arg);
 format_arg(Arg, binary) when is_binary(Arg) -> Arg;
 format_arg(Arg, string) when is_list(Arg) -> Arg;
 format_arg(Arg, string) when is_binary(Arg) -> binary_to_list(Arg);
+format_arg(undefined, binary) -> <<>>;
+format_arg(undefined, string) -> "";
 format_arg(Arg, Format) ->
     ?ERROR_MSG("don't know how to format Arg ~p for format ~p", [Arg, Format]),
     throw({error_formatting_argument, Arg, Format}).