What is the correct way to structure a ifelse to be able to filter using a parameter?

I am trying to use a parameter to create a set of records that can be filtered. I've created a parameter audience with the options of retrieving all the audience or a subset of registers from the register column. The filter is created with the below calculations with the following ifelse but the visual is giving me a type error (CONTEXTUAL_INVALID_ARGUMENT_TYPE). Can someone advise on the best way to do so?

ifelse(${audience}='all', 1=1,${audience}='registrants', register=1, 1=1)