Xendesktop - Machine Cration Service Event 2006

Disk tidying workflow operation failed : System.ArgumentNullException: Value cannot be null.
Parameter name: source ---> System.ArgumentNullException: Value cannot be null.
Parameter name: source
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at Citrix.PoolManagement.VMManager.VmmImplementation.Vmware.VmwareVmConnection.WaitForTask(ManagedObjectReference taskRef, TimeSpan timeout, Action`1 reportPercentComplete, TimeSpan taskPollPeriod)
   at Citrix.PoolManagement.VMManager.VmmImplementation.Vmware.VmwareVmManager.DeleteDisk(DiskImage diskImage)
   at HypervisorsCommon.HCL.TaskRunner`1.Run()
   --- End of inner exception stack trace ---
   at HypervisorsCommon.HCL.TaskScheduler`1.CompleteTask(IAsyncResult result)
   at Citrix.PoolManagement.VMManager.VmmImplementation.Vmware.VMwareHypervisor.EndDeleteDisk(IHostingUnitConnector hostingUnit, IAsyncResult result)
   at Citrix.MachineCreation.Logic.DeleteDisk(PendingDiskDelete diskToDelete)
   at Citrix.MachineCreation.PendingDiskDeletion.PendingDiskDeletionWorkflow.DoDeleteDisk(Object sender, EventArgs e)
   at System.Workflow.ComponentModel.Activity.RaiseEvent(DependencyProperty dependencyEvent, Object sender, EventArgs e)
   at System.Workflow.Activities.CodeActivity.Execute(ActivityExecutionContext executionContext)
   at System.Workflow.ComponentModel.ActivityExecutorOperation.Run(IWorkflowCoreRuntime workflowCoreRuntime)
   at System.Workflow.Runtime.Scheduler.Run()