User macro with enum parameter and multiple=true not working

Simple user macro allows a user to select a variable from list works fine when user wants to only select one item from the dropdown. However, when he try to add the multiple eattribute to allow users to select more than one item from list, macro fails. 

Problem

As you can see below, when you create a new User Macro without the Multiple properties, it will show up like a menu with a single selection:

But, when the "Multiple" property is included and set to "True" inside the User Macro, there is no way to select multiple values. There is not even a dropdown to select the values: