Operator List empty in a workflow process of a custom entity

Hi

In a workflow that I am creating based on a custom entity, I have a step where I try to update some field of the entity. In the form that opens on clicking on the 'Set Properties', the operator list empty.The list is empty on all attributes of the entity. On the same step, if I change the entity, the operator list gets populated.

Any idea why the operator list can be empty for an entity?