How do I click an element in PhantomJS?

page.evaluate(function() {
    document.getElementById('idButtonSpan').click();  
});

This gives me an error "undefined is not a function..."

If I instead

 return document.getElementById('idButtonSpan');

and then print it,

then it prints [object object], so the element does exist.

The element acts as a button, but it's actually just a span element, not a submit input.

I was able to get this button click to work with Casper, but Casper had other limitations so I'm back to PhantomJS.