Error using the Visitor ID to upload data using the Generic Data Source (Transaction ID)

We are trying to upload offline data linked to an online visit and attribute that as a success event to an online campaign. To do that we collect the Visitor Id during a session and then we try to use that to import the data through a Generic Data Source (Transaction ID).

This is a request example using the API 1.3 BeginDataBlock Method:

{
    "blockName":"test",
    "columnNames":[
        "Date",        
        "visitorID",
        "Evar 1"
    ],
    "dataSourceID":"1",
    "endOfBlock":"1",
    "reportSuiteID":"XXX",
    "rows":[
            "04/11/2015/16/30/00",
            "2B1CC581852C22E8-400000C0A013CFDD",
            "Test"
    ]
}

Based on this article this should be possible https://marketing.adobe.com/resources/help/en_US/sc/datasources/adobe_analytics_data_sources.pdf. However, when we try to do that we receive the following error during the data processing: "Column header: "visitorid" is not a valid column header"

Any idea of what might be causing this? Is this a known issue?

Thanks