BIGINT / STRING error when trying to fetch data from another form

BIGINT / STRING error when trying to fetch data from another form

I have a situation where I need to populate a text field (Assign_To_Email) with data based on a selection made in a dropdown (Assign_To). These are both in a form called Support_Tracking. 

The values in the Assign_To dropdown in Support_Tracking correspond to values in another dropdown also called Assign_To, but this one is in a form called Names_and_Emails that includes one other field, Assign_To_Email (a single line text field).

When someone selects a value in the Assign_To field in Support_Tracking, I want the application to look for the same value in the Assign_To field in Names_and_Emails, find the Assign_To_Email associated with it, and insert this in the  Assign_To_Email  field in Support_Tracking. 

I put this code in the Assign_To > On User Input section of the Support_Tracking form:
       temp = Names_and_Emails [ID == input.Assign_To];
      input.Assign_To_Email = temp.Assign_To_Email;

However, I get this error:  In Criteria left expression is of type BIGINT and right expression is of type STRING and the operator == is not valid

Can anyone help me sort this out?