What’s the correct way of submitting a transaction using cardano-graphql?

I’m getting this error when trying to submit a transaction using graphql:

{
  "errors": [
    {
      "message": "Deserialization failed in Transaction.metadata because: Expected null, found other type",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "submitTransaction"
      ],
      "extensions": {
        "code": "INTERNAL_SERVER_ERROR",
        "exception": {
          "stacktrace": [
            "Error: Deserialization failed in Transaction.metadata because: Expected null, found other type",
            "    at Object.<anonymous> (/app/packages/api-cardano-db-hasura/dist/executableSchema.js:77:43)",
            "    at Generator.throw (<anonymous>)",
            "    at rejected (/app/packages/api-cardano-db-hasura/dist/executableSchema.js:6:65)",
            "    at processTicksAndRejections (internal/process/task_queues.js:95:5)"
          ]
        }
      }
    }
  ],
  "data": null
}

What is this trying to say?

The transaction has been built using cardano-serialization-lib.

I’m 100% sure the transaction is correct because it’s been accepted using other methods.