"/fill ... replace ..." doesn't parse correctly for blocks that support data tags

I execute the command: /fill ~ ~ ~ ~ ~ ~ sand 0 replace stone
and I get: 0 blocks filled - so a success.
I execute the command: /fill ~ ~ ~ ~ ~ ~ hopper 0 replace stone
and it says: Data tag parsing failed: Unable to locate name/value seperator for string: stone
so it fails, even though I only changed sand to hopper !