Analyzing heap dump, Map#size() is not a function?

Question

Getting this weird error:

javax.script.ScriptException: sun.org.mozilla.javascript.internal.EcmaError: TypeError: size is not a function, it is null. (#1)

While analyzing a heap dump and running this OQL query on VisualVM:

select { map: x } 
from java.util.concurrent.ConcurrentHashMap x 
where x.size() < 10

The problem lies on the where clause, somehow it's not working though Map obviously has a size method.

Answer 1

@ruakh's answer is preety good, except one little thing. A segment sometimes could be null, which messes up sum(x.segments, 'it.count'). Replace it with

sum(x.segments, 'it != null ? it.count : 0')

and it will work fine. Tested upon my word.