The \"data\" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received undefined

[morgler]

I get this error:

{
  "errorType": "TypeError",
  "errorMessage": "The \"data\" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received undefined",
  "trace": [
    "TypeError [ERR_INVALID_ARG_TYPE] [ERR_INVALID_ARG_TYPE]: The \"data\" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received undefined",
    "    at Hmac.update (internal/crypto/hash.js:82:11)",
    "    at Object.hmac (/var/task/node_modules/aws-sdk/lib/util.js:423:50)",
    "    at Object.getSigningKey (/var/task/node_modules/aws-sdk/lib/signers/v4_credentials.js:74:35)",
    "    at V4.signature (/var/task/node_modules/aws-sdk/lib/signers/v4.js:98:36)",
    "    at V4.authorization (/var/task/node_modules/aws-sdk/lib/signers/v4.js:93:36)",
    "    at V4.addAuthorization (/var/task/node_modules/aws-sdk/lib/signers/v4.js:35:12)",
    "    at signRequest (/var/task/node_modules/@crft/appsync-gateway/dist/credentials.js:20:12)",
    "    at IAMCredentialsStrategy.sign (/var/task/node_modules/@crft/appsync-gateway/dist/credentials.js:42:16)",
    "    at GraphQLGateway.runQuery (/var/task/node_modules/@crft/appsync-gateway/dist/gateway.js:28:44)",
    "    at module.exports.handler (/var/task/src/PostConfirmation.js:40:19)"
  ]
}

I use the IAM strategy and I verified that all attributes are set (key, secret, token and region).

My lambda tries to call a mutation like so:

const createUser = `
  mutation createUser($input: CreateUserInput!) {
    createUser(input: $input) {
      id
      name
      email
    }
  }
`
const newUser = <someUserData>

await gateway.runQuery({
      operationName: 'createUser',
      query: createUser,
      variables: {
        input: newUser
      }
    })

Am I missing something? To me it looks like the signing doesn't work, because AWS signer expects different parameters.