GroupIntoBatches for non-KV elements

According to the Apache Beam 2.0.0 SDK Documentation GroupIntoBatches works only with KV collections.

My dataset contains only values and there's no need for introducing keys. However, to make use of GroupIntoBatches I had to implement “fake” keys with an empty string as a key:

static class FakeKVFn extends DoFn<String, KV<String, String>> {
  @ProcessElement
  public void processElement(ProcessContext c) {
    c.output(KV.of("", c.element()));
  }
}

So the overall pipeline looks like the following:

public static void main(String[] args) {
  PipelineOptions options = PipelineOptionsFactory.create();
  Pipeline p = Pipeline.create(options);

  long batchSize = 100L;

  p.apply("ReadLines", TextIO.read().from("./input.txt"))
      .apply("FakeKV", ParDo.of(new FakeKVFn()))
      .apply(GroupIntoBatches.<String, String>ofSize(batchSize))
      .setCoder(KvCoder.of(StringUtf8Coder.of(), IterableCoder.of(StringUtf8Coder.of())))
      .apply(ParDo.of(new DoFn<KV<String, Iterable<String>>, String>() {
        @ProcessElement
        public void processElement(ProcessContext c) {
          c.output(callWebService(c.element().getValue()));
        }
      }))
      .apply("WriteResults", TextIO.write().to("./output/"));

  p.run().waitUntilFinish();
}

Is there any way to group into batches without introducing “fake” keys?