evalXPath not defined in Truclient??

I am trying to grab all of the listbox elements on a given page as identified by the //*[@class=\"gwt-ListBox\"] xpath

Using the TruClient Help File Examples I am trying the below, only to get the error message "** failed - exception occurred: ReferenceError: evalXPath is not defined", any ideas why?

Please note that I've even copied down the specific  evalXPath call used in the example in order to ensure my syntax is correct, to no avail:

//Retrieve lit of elements matching the target object description
//var checkboxes = evalXPath("(//input[@type=\"checkbox\"])");
var listElements = evalXPath("(//*[@class=\"gwt-ListBox\"])");

var numberFound = listElements.length;

window.alert(numberFound);