Can't toggle custom field value though workflow process

We have a workflow rule set to approve all new project requests and then convert them into a project within the portfolio. All of a sudden we are getting the following error 'An error occurred while executing action '1 - Update Field' in a configuration named 'Set Contains Approval': Field Milestone of object T-42726 from class Task is empty. ' The 'Set Contains Approval' seems to check out, no syntax errors. I'm not sure what the Field Milestones of Object .... from class Task is empty actually means or where to investigate that. Any possible suggestions?