CRM 2011: Javascript: Object doesn't support property or method 'setSrc'

Hi there,

I've a simple Javascript which sets an image in an iFrame on change of a field. Here is the code used:

Xrm.Page.ui.controls.get("IFrame_Name").setSrc('/WebResources/SampleImage.png');
Object doesn't support property or method 'setSrc'

Tried this as well but no luck
Xrm.Page.getControl("IFrame_Name").setSrc('../WebResources/NoImage.png'); 
Object doesn't support property or method 'setSrc'

Any idea about this?

Thanks,
Abhi