Trying to query by equals throws general_query_invalid exception

I'm trying to query my collection filtered by a contactId. This contact id references ids of items in another collection. By the documentation something like this:

var documents = _databases.listDocuments(
        databaseId: constants.databaseId,
        collectionId: constants.CollectionIds.feedEntries,
        queries: [Query.equal("contactId", contactId)]);

Should do the job fine, as the contactId attribute exists on the feedEntries collection and contactId is a String provided from other parts of the app.

But the code above throws this:

What am I missing?