Can't get around this error after adding second dataset...A scope is required for all aggregates used outside of a data region unless the report contains exactly one dataset

I added a dataset to an existing report and broke an aggregation.  In the old (i.e. single dataset) report, this expression below worked fine.  I wanted to get a distinct count of the vst_ext_id field when my educated field was like "VTE1*"

= CountDistinct(IIF(Fields!educated.Value like "VTE1*", Fields!vst_ext_id.Value, Nothing))

After adding a new dataset, this no longer works and I get the error " A scope is required for all aggregates used outside of a data region unless the report contains exactly one dataset".  Having done some research online, I found that I needed to specify my dataset explicitly and I thought this new expression might work, but still no success...

= CountDistinct(IIF(Fields!educated.Value,"DataSet1" like "VTE12*", Fields!vst_ext_id.Value,"DataSet1", Nothing))

Am I missing something?  Based on online responses, this explicit dataset naming convention seems to help most people, but it isn't working for me. 

Thanks in advance!

Brian