Adding expression function to array plugin

I have attempted to add a new expression function to the Array "plugin", but having a hard time making Construct understand that the expression is taking one parameter.

I've added the following code to runtime.js:

     Exps.prototype.rank = function (ret, playerNumber)

     {

          var ranking = 1;

            //Do whatever...

          ret.set_int(ranking);

     }

And added the following to edittime.js:

AddExpression(13, ef_return_number, "Get rank for player", "Array", "rank", "Get rank for a player");

The expression shows up in Construct as expected (i.e. I can enter SomeArray. and the "rank" expression will show up in the dropdown menu). Though, if i enter SomeArray.rank(1), it says 'rank' does not take 1 parameter (should have 0).

What am I doing wrong? How do I properly make the expression take a parameter?