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Hi, I'd be happy to work on this issue

I've played a little around this issue. I can reproduce the behaviour in a unit simple test, using just the plain CEL library.

I found this in the CEL lib documentation ( here ):

The null value is written null. It is used in conversion to and from protocol buffer and JSON data, but otherwise has no built-in meaning in CEL. In particular, null has its own type (null_type) and is not necessarily allowed where a value of some other type is expected.

I didn't manage to compare anything (the name key in the example below) to null. Using an expression like record.value.name == null to validate on a map like this :

Map<String, Object> objectKeys = new HashMap<>();
objectKeys.put("name", null);
objectKeys.put("age", 24);
Map<String, Object> valueKeys = new HashMap<>();
valueKeys.put("value", objectKeys);
Map<String, Map<String, Object>> recordKeys = new HashMap<>();
recordKeys.put("record", valueKeys);

Gives me an output result like this:

unknown {
  exprs: 3
}

Do you know a specific way to do this kind of comparison using this library?

Can you send a draft PR with your progress so far? I'd like to try/experiment what the change looks like along with you

I didn't manage to compare anything

yeah me neither, I've tried typeof and some other things, nothing worked (which worked fine here: https://playcel.undistro.io/ ). Most likely, our CEL env requires adjustments (see MessageFilters#createCompiler and MessageFilters#createRuntime).

Hi,
Here is my PR.
It is just a draft, since it doesn't work ;)
I've added multiple tests, one using the CEL as it is setup in the app. And two others completely outside the app, to see if there was something special in the setup.
Testing String and Number equality and builtin functions works but not number inequality nor null checks