Batching multiple files to Amazon S3

I'm trying to upload multiple files to Amazon S3 all under the same key, by appending the files. I have a list of file names and want to upload/append the files in that order. I am pretty much exactly following this tutorial but I am looping through each file first and uploading that in part. Because the files are on hdfs (the Path is actually org.apache.hadoop.fs.Path), I am using the input stream to send the file data. Some pseudocode is below (I am commenting the blocks that are word for word from the tutorial):

try {
      int i = 0;
      for (String file : files) {
        Path filePath = new Path(file);

        // Get the input stream and content length
        long contentLength = fss.get(branch).getFileStatus(filePath).getLen();
        InputStream is = fss.get(branch).open(filePath);

        long filePosition = 0;
        while (filePosition < contentLength) {
            // create request
            //upload part and add response to our list
            i++;
        }
    }
    // Step 3: Complete.
    CompleteMultipartUploadRequest compRequest = new
          CompleteMultipartUploadRequest(bk.getBucket(),
          bk.getKey(),
          initResponse.getUploadId(),
          partETags);

    s3Client.completeMultipartUpload(compRequest);
} catch (Exception e) {
      //...
}

}

However, I am getting the following error:

com.amazonaws.services.s3.model.AmazonS3Exception: The XML you provided was not well-formed or did not validate against our published schema (Service: Amazon S3; Status Code: 400; Error Code: MalformedXML; Request ID: 2C1126E838F65BB9), S3 Extended Request ID: QmpybmrqepaNtTVxWRM1g2w/fYW+8DPrDwUEK1XeorNKtnUKbnJeVM6qmeNcrPwc
    at com.amazonaws.http.AmazonHttpClient.handleErrorResponse(AmazonHttpClient.java:1109)
    at com.amazonaws.http.AmazonHttpClient.executeOneRequest(AmazonHttpClient.java:741)
    at com.amazonaws.http.AmazonHttpClient.executeHelper(AmazonHttpClient.java:461)
    at com.amazonaws.http.AmazonHttpClient.execute(AmazonHttpClient.java:296)
    at com.amazonaws.services.s3.AmazonS3Client.invoke(AmazonS3Client.java:3743)
    at com.amazonaws.services.s3.AmazonS3Client.completeMultipartUpload(AmazonS3Client.java:2617)

If anyone knows what the cause of this error might be, that would be greatly appreciated. Alternatively, if there is a better way to concatenate a bunch of files into one s3 key, that would be great as well. I tried using java's builtin SequenceInputStream but that did not work. Any help would be greatly appreciated. For reference, the total size of all the files could be as large as 10-15 gb.