Pre-selecting checkboxes in a wufoo form

I am having some trouble pre-selecting checkboxes on an iframe wufoo form using a query string. I followed the query string format in this article: http://ift.tt/1Cn7RfB

I was able to successfully pre-populate input boxes using http://ift.tt/1ErjYar as an example, but if I am not able to do the same for check boxes. For instance I tried /def/field1=true but that did not work. I seem to be missing something. Any ideas?

Thanks!