When designing a workflow with a Set Data Field event and using the Email property of a Person or Group Item Reference to set one of the fields, using the data field as the To field in an Email event results in the workflow instance going into an error state. This scenario is also valid when using the Email property of a Created By column Item Reference to set a data field.
Another scenario is when using the Get List Item Reference event and then the Set Data Field event. In this scenario the workflow instance will also go into an error state but with a slightly different error message.
The error message for the first scenario will state that: “The email address : System.String, is invalid”.
The error message for the second scenario will state that: “Object of type 'System.Object' cannot be converted to type 'System.String'”.