Can't transform a purchase order to a vendor return authorization?

Getting an error when attempting to transform a purchase order to a vendor return authorization. Anyone else seen this?

INVALID_REF_CANT_INITIALIZE - You can not initialize vendorReturnAuthorization by referencing purchaseOrder.

Here's my code (C#):

 InitializeRecord initRecord = null;    VendorReturnAuthorization record = null;    initRecord = new InitializeRecord  {        type = InitializeType.vendorReturnAuthorization,        reference = new InitializeRef { externalId = purchaseOrder.externalId, type = InitializeRefType.purchaseOrder, typeSpecified = true }  };    ReadResponse response = service.initialize(initRecord); 

SuiteScript documentation says it's a supported transform (though I haven't actually tried it), but maybe it's not supported in SuiteTalk yet. I'll probably submit a support case, but thought I'd check here first to make sure it isn't something obvious I'm missing.