Echo sign on Custom Object not relating to Contacts, Opportunites, Lead

Hi

I am creating new object to use Echo sign . Its a custom Objects 1 which looks up to another Custom object 2 (Parent Object).

I have created the document template and saved in library

Created the agreement Template in Echosign ,

While adding recipients in the related list , when i select look to Master Object I am getting the following error.

"No fields of type Contact found on the master object."

Please help me on this