$filter not returning expected data

Hi there,

I using this query below, but instead returning the 'completedDataTime' where the value is null, it keeps returning, all the plan, even that one who has the 'completedDateTime' not null. I just started using Graph API, but I don't think I'm doing something wrong. Just be just simple as that.

https://graph.microsoft.com/v1.0/planner/plans/{plan-id}/tasks?$filter=completedDateTime eq null

For those who has more exprience on me, am I doing something wrong or missing something on my query?

Cheers,