The notifyToStartChangeStreamsMonitor calls ReshardingDonorService::DonorStateMachine::createAndStartChangeStreamsMonitor which uses _cancelableOpCtxFactory. Currently, notifyToStartChangeStreamsMonitor is called immediately after the DonorStateMachine so the _cancelableOpCtxFactory might not have been initialized yet because run() is called asynchronously. This is a test-only issue since in a non-test setup createAndStartChangeStreamsMonitor (i.e. _shardsvrReshardingDonorStartChangStreamsMonitor) is by design called after the donor is already in the "donating-initial-data" state, therefore has already initialized _cancelableOpCtxFactory.