Filter Expression can only contain non-primary key attributes

Question

I'm new to AWS Amplify, and have created a GraphQL API for a project that I'm working on in order to learn it & AppSync. Essentially I have a schema that looks like the following:

type User @model {
  id: ID! @primaryKey
  boards: [Board] @hasMany
  createdAt: String!
  updatedAt: String!
}

type Board @model {
  id: ID! @primaryKey
  createdBy: User!
  title: String!
}

and I'm trying to run the following query in the AppSync console:

query MyQuery {
  listUsers {
    items {
      boards {
        items {
          title
        }
      }
    }
  }
}

But for some reason I keep seeing this error:

Filter Expression can only contain non-primary key attributes: Primary key attribute: userBoardsId

I've specified the primary key in both models, and I'm aware that AppSync generates the 'userBoardsId' foreign key, but I'm not sure why it's causing an issue.