Cannot save the workflow because it is not locked.

Hello All,

We get below the error when processing Purchase order workflow and the error doesn't have any issues with the x++ codes and we tried reprocessing the workflow and it went fine. Is there any specific reason for the erorr or how can we avoid such error in the future? Any suggestions would be a great help !!!

Workflow Instance ID: 006891 System.ServiceModel.Internals  SysWorkflowQueue-resume  SysWorkflow-save X++ Exception: Cannot save the workflow because it is not locked.
 at SysWorkflow-save
SysWorkflowQueue-resume

    at System.Runtime.AsyncResult.End[TAsyncResult](IAsyncResult result)
   at System.Runtime.DurableInstancing.InstancePersistenceContext.EndOuterExecute(IAsyncResult result)
   at System.Activities.WorkflowApplication.UnloadOrPersistAsyncResult.OnPersisted(IAsyncResult result)
   at System.Runtime.AsyncResult.SyncContinue(IAsyncResult result)
   at System.Activities.WorkflowApplication.UnloadOrPersistAsyncResult.Track()
   at System.Activities.WorkflowApplication.UnloadOrPersistAsyncResult.InitializeProvider()
   at System.Activities.WorkflowApplication.UnloadOrPersistAsyncResult..ctor(WorkflowApplication instance, TimeSpan timeout, PersistenceOperation operation, Boolean isWorkflowThread, Boolean isInternalPersist, AsyncCallback callback, Object state)
   at System.Activities.WorkflowApplication.IdleEventHandler.OnStage1Complete(IAsyncResult lastResult, WorkflowApplication application, Boolean isStillSync)
   at System.Activities.WorkflowApplication.IdleEventHandler.Run(WorkflowApplication instance)

/Uday.