Accessing Multi-valued Parameters

In my report (SSRS 2012) I have a multi-valued parameter (@BankNum) that is populated by a list of available banks.

In the dataset (parameters tab) I have the expression = join(Parameters!BankNum.Value, ""), and the parameter is set as a multi-valued parameter under the report parameter area.

When the report runs for a single value, the data appears as expected. When multiple or all is selected, the report comes back empty.

What am I missing?

Thanks!