Using subreport binding to set ReportSource

I am using Telerik Report Designer, and I have a report that has many nested subreports. The report works fine, but I am trying to change one subreport based on user input.  I attempted to do this by adding a Binding to ReportSource like this:

= IIf(Parameters.Images.Value,"ShowImages.trdx", "NoImage.trdx")

When I run this I get the error

I've tried different variations of the Expression, including the full path of the trdx file,  but get the same error. Is there a way to change which file is referenced?

Thanks,

Tracy