Why --type-set doesn't work in ack-grep

Why --type-set doesn't work in ack-grep

In my .ackrc file I have this line
--type-set=vm=.vm
But when I run
ack-grep --vm blah
it searches all types of files.
Neither does this work:
ack-grep blah --type-set vm=.vm
ack-grep --type-set vm=.vm blah
Why?