Reading BigQuery federated table as source in Dataflow throws an error

I have a federated source in BigQuery which is pointing to some CSV files in GCS.

When I try to read to the federated BigQuery table as a source for a Dataflow pipeline, it throws the following error:

    1226 [main] ERROR com.google.cloud.dataflow.sdk.util.BigQueryTableRowIterator  - Error reading from BigQuery table Federated_test_dataflow of dataset CPT_7414_PLAYGROUND : 400 Bad Request
{
  "code" : 400,
  "errors" : [ {
    "domain" : "global",
    "message" : "Cannot list a table of type EXTERNAL.",
    "reason" : "invalid"
  } ],
  "message" : "Cannot list a table of type EXTERNAL."
}

Does Dataflow not support federated sources in BigQuery, or am I doing something wrong? I do know that I could read the files from GCS directly into my pipeline, but I'd prefer to work with BigQuery TableRow objects instead due to the design of the application.

 PCollection<TableRow> results = pipeline.apply("fed-test", BigQueryIO.Read.from("<project_id>:CPT_7414_PLAYGROUND.Federated_test_dataflow")).apply(ParDo.of(new DoFn<TableRow, TableRow>() {
        @Override
        public void processElement(ProcessContext c) throws Exception {
            System.out.println(c.element());
        }
    }));