Hierachy Workflow Error

I keep getting this error message when I use my purchase requisition workflow;

Stopped (error): X++ Exception: The hierarchy provider did not return any users.

at SysWorkflowHierarchyProvider-resolve

SysWorkflowHierarchyProvider-resolveHierarchy

SysWorkflowQueue-resume

Pls how do I resolve this? The thing is, the hierachy is flowing but wen it gets to the managing director, who is the last person on the hierachy, once he approves, it throws this error message.