I have a custom picklist for Tasks and I am trying to create a workflow that will update the Subject field with the value from this custom picklist field when create or save record.

The problem I have is that I am getting error for the Subject field formula in the field update rule.
in the formula field I type custom field name which is Type_Value__c and get this error:

Error: Formula result is data type (Text), incompatible with expected data type (Picklist).

Could you help me?