Prismic Graphql `where` filter is not working as expected

i want to get the list of posts under a category. The category is a CustomType and i made Content Relationship between a post and a category.
Let the category has one label (Text), uid(UID) , 2 Slices nested category

When i query as below i get result but not what I needed & all otherways i tried not working. What is the exact way to filter a post(CustomType) with category(CustomType)

query {
AllPosts(where: {category: null}) {
totalCount
}
}

i get all post with this but when i try to replace null with string i get no results. Just empty ! .

What String should i use in place of null

Hello @wygininc, can you share the actual query you're running in your repository's GraphQL endpoint?
That way, we can also try and run a test.
If you prefer you can send this via dm.