Passing NULL into a Procedure in Preview Causes Exception

I have a virtual procedure that accepts one parameter that may be null. When I pass a value in it works fine but when I check the NULL checkbox fir the input variable I get the following exception, "Remote org.teiid.api.exception.query.QueryValidatorException: Bind variables (represented as "?") were found but are allowed only in prepared or callable statements."

Here is the generated query from preview: select * from ( exec RestfulBooks.getBooks ) AS X_X