Workflow Error Message: Value for argument "previousFlowElement" must be specified.

I'm getting this error message for a workflow with a simple trigger.

Terrasoft.Common.ArgumentNullOrEmptyException: Value for argument "previousFlowElement" must be specified.

   at Terrasoft.Core.Process.FlowParallelGateway.Accept(FlowVisitor visitor, FlowElement previousFlowElement)

   at Terrasoft.Core.Process.FlowVisitor.AcceptFlowElement(FlowVisitorQueueItem queueItem)

I trigger the workflow as a sub-process and pass the Opportunity ID. For test purposes I trigger the flow directly from the opportunity which is when I get the above error message.