What is causing ORA-00911 when using a UDQ within an expression

Summary:

Client wants to get the number of transactions where the value on the transaction attribute6 field match. When writing the expression we are getting an invalid expression due to ORA-00911: invalid character -Failure. For the count value set function does the value set need to return a number value or will character work too?

Content (please ensure you mask any confidential information):

It appears that we are passing the correct field criteria (Number or VARCHAR) for the appropriate field. For the function count_value_set, do you have to pass a numeric value in order to count? Or is there something else we are overlooking.