Seems like in 2.5 there is a difference in matching null fields for arrays.

Quick repro of issue:

db.test.insert({a: [{b:1}, {c:1}]})
db.test.find({"a.b": null})

document is not found in 2.4.8, but is found in 2.5.5-pre (built morning of 11/19)

The way to think about this is that "there is some element in the array such that the predicate is true." In the example above, there is a "path" such that a.b is null and that is the sub-document element

.