Was writing a custom workflow activity on incident entity and in which we wanted the Customer Lookup Value. And based on type of Customer i.e. either contact or account there was some logic to be executed.
So declared an input parameter or type Customer
However while registering the workflow I got the below error
The way to handle this is,
Get the guid of the record from the context and then retrieve the value of customer id attribute.
On retrieval check if the EntityReference is of type account or contact and then proceed accordingly
Hope it helps ..