At the 27.4.0 docker engine version, there was a change with the "blkio_stats. io_service_bytes_recursive.op" in the container stats API response. The value of the "op" became lowercase, and it caused a mismatch in the getIOBytes function which is called with Capitalised op.
Example:
getIOBytes(container.get_child("blkio_stats.io_service_bytes_recursive"),"Write");