aws cdk appsync Schema Creation Status is FAILED with details: Internal Failure while saving the schema

Given the following graphql schema

# graphql/schema.graphql
type AppUser {
  userId: String
  fullName: String
}

type Query {
  getUser(userId: String): AppUser
  getUsers(): [AppUser]
}

type Mutation {
  addUser(user: AppUser!): AppUser
}

And the following cdk code defined at lib/mystack.ts:

    const graphql = new appsync.GraphQLApi(this, 'GraphQLApi', {
      name: 'metrolens-graphql-api',
      logConfig: {
        fieldLogLevel: appsync.FieldLogLevel.ALL,
      },
      authorizationConfig: {
        defaultAuthorization: {
          // userPool,
          defaultAction: appsync.UserPoolDefaultAction.ALLOW,
        },
        // additionalAuthorizationModes: [{ apiKeyDesc: 'My API Key' }],
      },
      schemaDefinitionFile: props?.schemaDirectory,
    })

    const lambdaRegister = new nodejs.NodejsFunction(this, 'register', {
      functionName: 'register',
      runtime: lambda.Runtime.NODEJS_12_X,
      timeout: cdk.Duration.seconds(10),
      entry: './lambda/register/register-0.ts',
      handler: 'handler',
      layers: [layer],
      description: 'Register a user.',
    })

    const lambdaLogin = new nodejs.NodejsFunction(this, 'login', {
      functionName: 'login',
      runtime: lambda.Runtime.NODEJS_12_X,
      timeout: cdk.Duration.seconds(10),
      entry: './lambda/login/login-0.ts',
      handler: 'handler',
      layers: [layer],
      description: 'Login a user.',
    })

    const lambdaRegisterDataSource = graphql.addLambdaDataSource(
      'lambdaRegister',
      'Register lambda triggered by appsync',
      lambdaRegister
    )
    lambdaRegisterDataSource.createResolver({
      typeName: 'Mutation',
      fieldName: 'addUser',
      requestMappingTemplate: appsync.MappingTemplate.lambdaRequest(),
      responseMappingTemplate: appsync.MappingTemplate.lambdaResult(),
    })

    const lambdaLoginDataSource = graphql.addLambdaDataSource(
      'lambdaLogin',
      'Login lambda triggered by appsync',
      lambdaLogin
    )

    lambdaLoginDataSource.createResolver({
      typeName: 'Query',
      fieldName: 'getUsers',
      requestMappingTemplate: appsync.MappingTemplate.lambdaRequest(),
      responseMappingTemplate: appsync.MappingTemplate.lambdaResult(),
    })

I get the following error message during the AWS::AppSync::GraphQLSchema deployment phase:

GraphQLApi/Schema (GraphQLApiSchema5937B126) Schema Creation Status is FAILED with details: Internal Failure while saving the schema.

Has anyone encountered this error before? I suspect there is something wrong with my graphql schema but I couldn't tell what it was. Another answer mentioned that the DynamoDB reserved words at https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/ReservedWords.html could cause conflicts, even if you don't use DynamoDB, so I renamed the User type to AppSync. However, it looks like Query is also a reserved word, but as far as I understand, that is also necessary for a graphql definition. Please help. AWS CDK suck hard.

The issue was my schema was incorrect. AppUser is a type, but I used as an input for the addUser mutation. You define the inputs for queries and mutations with the special input keyword. I ended up duplicating AppUser to be:

input AppUserInput {
  userId: String
  fullName: String
}

And that solved the problem.