invalid_parts_field when performing a commit upload

I am developing a file uplaod routine and for the large files, I am using Chunked Uploading.
Everything "seems" to be workgin as per the developer documentation, however, when I execute the final commit I am getting an invalid_parts_field error as the response.
I have checked the data I am posting up and it seems to be correct. - the offsets are correctly calculated, as is theSHA-1 digest etc.

The file is quite large and is broken into 22 chunks.