Testforblocks not working

When player uses the testforblocks command, whether or not the two locations are identical the console says they're not.

For example:

/execute @p ~ ~1 ~ testforblocks 0 4 0 0 100 0 ~ ~ ~

or even simply:

/testforblocks 13 6 -4 17 6 -8 18 6 -4 22 6 -8