Error when removing a resource from a resource group

Hi,

Is anyone familiar with this error we are receiving when trying to delete any resource from a few specific groups, but not all groups,  under the PERSONS tab in Resource Details?  We have checked/compared all the set up and we are clearly missing something.  

 

ResourceDetails/PersonGroupForm (server error)

Database error occurred. Contact administrator.

Error details: [{"code":1400,"message":"ORA-01400: cannot insert NULL into (\"NORT1APP\".\"RESOURCE_AVAIL_SHIFT_TAB\".\"SHIFT_END\")"}]

Thanks,

Jo