Set originalReferrer via API

I'm trying to set the originalReferrer field via the REST API.

When I POST the following to https://anonymized.mktorest.com/rest/v1/leads.json:

{  
   "action":"updateOnly",
   "lookupField":"email",
   "input":[  
      {  
         "email":"test@anonymized.com",
         "firstName": "test",
         "originalReferrer": "http://www.test.com"
      }
   ]
}

Marketo responds with:

{
    "requestId": "[anonymized]",
    "result": [
        {
            "id": 123456,
            "status": "updated"
        }
    ],
    "success": true
}

Seems reasonable enough.

However, when I view the lead in Marketo, Original Referrer is not set, though First Name is set to "test".

Any thoughts?