There is a mistake in the method Not() of MongoDB.Driver.Builders.Query.

Line:
MongoDB.Driver\Builders\QueryBuilder.cs(513): return new QueryDocument(elementName, new BsonDocument("$ne", operatorValue));

Presumably there should be "$not" instead of "$ne".

Now a valid Where query { "$where" :

{ "$code" : "this.Length == null" }

} is converted by Not() into invalid { "$where" : { "$ne" :

{ "$code" : "this.Length == null" }

} } which fails with an error:

$where expression has an unexpected type (response:

{ "errmsg" : "exception: $where expression has an unexpected type", "code" : 15902, "ok" : 0.0 }

)

Driver version 1.8.3