WPGraphql Error: Unknown type "User" in Gridsome

Question

I'm using WPGraphQL with Gridsome. I am trying to query user avatar for post comments. Here is my query

comments(where: {order: ASC}) {
   edges {
     node {
        id
       author {
         node {
            __typename
            ... on User  {
              id
              avatar {
                url
              }
            }
         }
       }
     }
   }
}

This query works in WPGraphql IDE and returns avatar URL but I get an error when I try in Gridsome.

Error: Unknown type "User". 

Any idea why this error? Thanks.