Why am I being told that my 'then' and 'else' expressions are not the same type?

I used an if-then-else statement to define a variable in a query join on IBM Cognos Report Studio 10.2.2. In the 'then' clause, I use the hard-coded string 'Not reportable'. For the 'else' clause, I use the variable [Generational Distribution], which is defined elsewhere in the query join, and is a string-valued variable from one of the joining query. I would therefore expect that the 'then' and 'else' clauses are both string-valued.

However, when I run, I get the following error:

OP-ERR-0206 Unsupported 'if' expression dataItem = "Generation Reportable." The 'then'
(expression = "'Not reportable'") and 'else' (expression = "[Generational Distribution]")
clauses must have the same data type.

The details begin:

RSV-SRV-0042 Trace back:RSReportService.cpp(724): QFException: CCL_CAUGHT: