A count on a collection that exists yields an error on an invalid query. All is good.

> db.foos.insert(

{ bars: [ 1, 2 ]}

)
> db.foos.find({ bars : { $in : [ 1 ]}}).count()
1
> db.foos.find({ bars : { $in : null }}).count()
Mon Feb 11 16:45:53 uncaught exception: count failed:

{ "errmsg" : "exception: invalid query", "code" : 12580, "ok" : 0 }

A count on a collection that doesn't exist yields 0, which is, IMHO, incorrect.

> db.invalid.find({ bars : { $in : null }}).count()
0

This is an odd optimization, I would expect them to fail the same way.