Error when I convert to string value in Report Designer

Question

This has got to be simple but I cannot find an answer anywhere.

I'm using Report Designer 2012, running local IIS and WebViewer. The report works just fine outputting the data form the database just like I'd expect until I get to the summary.The report begins with a summary page and is followed with a list for subsequent pages.

I can successfully output expressions like =(2+3), =Code.ConcateString("val1", "val2") and =First(Fields!first_name.Value, "CallLog")

Anything like this fails:

=First(Fields!first_name.Value, "CallLog") & " test"

=Code.MyMethod(First(Fields!first_name.Value, "CallLog"))

=CStr(First(Fields!first_name.Value, "CallLog"))

So I think I've narrowed the obscure #ERROR tag to it having something to do with the string conversion.

My end goal is to simply output (Fields!first_name.Value, "CallLog") and (Fields!last_name.Value, "CallLog") to a single textbox.