Fill command & chests

typing the command "/fill x1 y1 z1 x2 y2 z2 chest 0 replace chest 0" yields the result: "Data tag parsing failed: Invalid tag encountered, expected '{' as first char."

I have tried many variations on the command including putting a "{}" in multitudes of different places and amounts. Nothing seems to work.

All I particularly wanted to do was replace chests with empty ones.
I am not sure if this was intended, or I am just doing it wrong, however it is worth reporting imo.

Thanks!
-Bc