Event handler problem

[rollOrDie]

Im trying to create a script that has its own built in event handlers, so that I can keep the HTML separate from the behaviour.

 

I have to <a> tags, with separate id's, but I keep getting the 'has no properties' error message?

 

Here is my code!

// hide graphic thumbnails
function hideGraphic (portThumbGraphic)
    {
        var portThumbGraphic = document.getElementById ('portThumbGraphic');
        portThumbGraphic.style.display = 'none';
    }

// hide website thumbnails
function hideWebsite (portThumbWebsite)
    {
        var portThumbWebsite = document.getElementById ('portThumbWebsite');
        portThumbWebsite.style.display = 'none';
    }

// hide graphic thumbnails by default
window.onload = hideGraphic;

var btnThumbWebsite = document.getElementById ('btnThumbWebsite');
var btnThumbGraphic = document.getElementById ('btnThumbGraphic');

// set functions to nav list
btnThumbWebsite.onclick = hideGraphic;
btnThumbGraphic.onclick = hideWebsite;

 

Any ideas? Thanks to anyone who can help.