When I try to call AlterTask method I get the error saying "Specified method is not supported"

SPWorkflowTask.AlterTask(this._TaskListItem, taskHash, true);

this._TaskListItem = SPList item
taskhash = properties that needs to be updated.

Any idea why it would be doing this? what should i do to not get this error.
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If am correct you are storing a reference to the List Item in your c# workflow.
SPListItem may not be serializable. Since the workflow serialized to the DB and then de-serializes - a reference to it may be lost.
Hence please try using the workflowItemProperties (object from OnWorkFlowInitiated method) object instead which contains a reference to the List item on which the current workflow is running.

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