_owner field can't be queried?

Hi,
I’m trying to query a collection to return only records with a matching _owner value.
This is a snippet from the backend function:

const {items} = await wixData.query('Survey')
            .eq('_owner', ownerId)
            .descending('_updatedDate')
            .find();

It always returns zero rows.
ownerId is a string, the _owner field on the Collection says it’s a string. (ownerId is passed into the function as a parameter)
If I query on any other field on the collection it works fine, so that should rule out authorisation issues. Just _owner has a problem being queried.

What am I missing?

thanks

duh. the passed in ownerId value was null. Ignore the question. _owner works fine…