'civicrm_grant' is not a valid option for field api_entity

Am just using this for the first time, and it seems to work nicely until the front-end grant application is submitted, when I get 'civicrm_grant' is not a valid option for field api_entity and progress no further (same if I choose 'Save as Draft').

I can't find civicrm_grant in API3 explorer - should I? Or is this issue with CiviGrant?

cheers, Nic