"Error transforming meta data transformation: java.lang.NullPointerException"

I get this error frequently in RM 5, when using a Join operator with the inner join type.

I have 2 example sets with IDs set correctly, using the inner join type.

I was never 100% sure but it seems like the inner join type produces a new example set with attributes from the intersection set of two ID attribute columns.

Any idea what this could be?

Thanks,
-Gagi