Report Builder Parameter Properties: Specify Available Values

Hoping this is the best place to post Report Builder questions.

My report ran well until I wanted to limit the available values. They all fall within the range I have supplied in the data. See the photo - I am free-typing in the Label Box, and then entering an expression for the value as shown.

Is that incorrect usage? Is there a better way to do this?

After doing that, I get this error on report run: 

'Microsoft.ReportingServices.ReportProcessing.ReportProcessingException' was thrown. ---------------------------- An error occurred during local report processing.