/clone technically still copies air blocks with mode=masking?

The 'masking' mode technically still copies air blocks, this is more misleading text than a bug?

e.g trying to /clone a 16x32x16 area with the mode=masking still results in a failure due to 8192 blocks being more than 4096, even if less than 4096 ( even less than 400 ) of the blocks are actually filled.