Why does my filter gets zero answers back( exam question: 4)

Hello,

IM trying to solve exam question 4 and I did this was working :

{"tripduration": {$elemMatch: [{"$exist": true, "$eq": null}] }}

but I get zero documents back and that cannot be the answer to that question.

So where did I go wrong ?

Roelof

you might be on the wrong cluster, or using the wrong database, or collection. may be you misspelled a field name.