Cost estimation for "not(@x)" calculates cost for "@x='value'" instead

The cost estimation for a query that uses a Lucene index calculates the cost incorrectly if there is a "not(<property>)" condition. Examples:

/jcr:root//*[(not(@x)) and (not(@y))

The Lucene query is then:

+:nullProps:x +:nullProps:y

But the cost estimation seems to take into account the number of documents for the fields "x" and "y", instead of the field ":nullProps"