Cardano graph QL utxo_aggregate sum error

Question

I am trying to get count of NFTs locked on some address by policy ID using cardano graph ql.

I have this query in cardano-graphql (https://github.com/input-output-hk/cardano-graphql):

query {
  utxos_aggregate(
    where: {
      _and: {
        address: { _eq: $address },
        tokens: {
          asset: {
            policyId: { _eq: "d6fe6efa7788cb70e57a91891605e3694352cabb4837e870610300e9"}
          }
        }
      }
    }
  ){ aggregate {
        sum {
        tokens {
          quantity
        }
        }
    }
  }
}

It works well with count aggregate, but in case of sum aggregate it returns this error:

{
  "errors": [
    {
      "message": "Cannot return null for non-nullable field Query.utxos_aggregate.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "utxos_aggregate"
      ],
      "extensions": {
        "code": "INTERNAL_SERVER_ERROR",
        "exception": {
          "stacktrace": [
            "Error: Cannot return null for non-nullable field Query.utxos_aggregate.",
            "    at completeValue (/app/node_modules/graphql/execution/execute.js:560:13)",
            "    at completeValueCatchingError (/app/node_modules/graphql/execution/execute.js:495:19)",
            "    at resolveField (/app/node_modules/graphql/execution/execute.js:435:10)",
            "    at executeFields (/app/node_modules/graphql/execution/execute.js:275:18)",
            "    at executeOperation (/app/node_modules/graphql/execution/execute.js:219:122)",
            "    at executeImpl (/app/node_modules/graphql/execution/execute.js:104:14)",
            "    at Object.execute (/app/node_modules/graphql/execution/execute.js:64:35)",
            "    at /app/node_modules/apollo-server-core/dist/requestPipeline.js:261:48",
            "    at Generator.next (<anonymous>)",
            "    at /app/node_modules/apollo-server-core/dist/requestPipeline.js:8:71"
          ]
        }
      }
    }
  ],
  "data": null
}

Do you have any idea how to fix this or what is wrong with my query?