Click event reports an error

I created an event that fires when the user clicks on the graph2d object.

Example:

   function onGraphChange ()
   {
      delayRenderUpdate = RENDER_STEP_SIZE*4;
   }
   graph2d.on('click',onGraphChange);

When it fires in the browser, I see the following error traceback:

DataAxis.screenToValue @ vis.js:26773 Graph2d.getEventProperties @ vis.js:25293 (anonymous function) @ vis.js:25090 etc.

Digging into the code, this corresponds to:

  DataAxis.prototype.screenToValue = function (x) {
    return this.scale.screenToValue(x); // problem is on this line
  };

I just started using vis.js today, so I'm not sure what is intended here.

UPDATE: Just discovered this happens regardless of whether I have an event defined or not. I deleted the event, reloaded (CTRL+F5) the view, and clicked on the graph2d. Same problem.