GraphQL Mutation Fails with “Missing Required Fields” Error in Amplify Gen 2

I’m working with AWS Amplify Gen 2 and defining a custom GraphQL schema using the @aws-amplify/backend package.

Here’s my schema definition:

import { a } from "@aws-amplify/backend";
import { SupplierInvoiceCreationFunction } from "../../../functions/SupplierInvoiceCreation/resource";

export const SupplierInvoiceCreationSchema = {
  CreateSupplierInvoice: a
    .mutation()
    .arguments({
      input: a.ref("SupplierInvoiceInput").required(),
    })
    .returns(a.ref("SupplierInvoiceCreationResponse"))
    .authorization((allow) => [allow.groups(["Suppliers"])])
    .handler(a.handler.function(SupplierInvoiceCreationFunction)),

  SupplierInvoiceInput: a.customType({
    InvoiceDate: a.string().required(),
    InvoiceDocument: a.ref("InvoiceDocumentsInput").array(),
  }),

  InvoiceItemDetailsInput: a.customType({
    InvoiceItemID: a.string().required(),
  }),

  InvoiceDocumentsInput: a.customType({
    FilePath: a.string().required(),
  }),

  SupplierInvoiceCreationResponse: a.customType({
    SupplierInvoiceStatus: a.string().required(),
  }),
};

Here’s the mutation I’m running:

mutation CreateSupplierInvoice {
  CreateSupplierInvoice(
    input: {
      InvoiceDate: "2025-11-01"
      InvoiceDetailsItem: [
        {
          InvoiceItemID: "ITEM-001"
        }
      ]
      InvoiceDocument: [
        {
          FilePath: "s3://bucket/invoices/invoice001.pdf"
        }
      ]
    }
  ) {
    SupplierInvoiceStatus
  }
}

But I’m getting the following error:

{
    "data": null,
    "errors": [
        {
            "path": null,
            "locations": [
                {
                    "line": 3,
                    "column": 5,
                    "sourceName": null
                }
            ],
            "message": "Validation error of type WrongType: argument 'input' with value 'ObjectValue{objectFields=[ObjectField{name='InvoiceDate', value=StringValue{value='2025-11-01'}}, ObjectField{name='InvoiceDetailsItem', value=ArrayValue{values=[ObjectValue{objectFields=[ObjectField{name='InvoiceItemID', value=StringValue{value='ITEM-001'}}]}]}}, ObjectField{name='InvoiceDocument', value=ArrayValue{values=[ObjectValue{objectFields=[ObjectField{name='FilePath', value=StringValue{value='s3://bucket/invoices/invoice001.pdf'}}]}]}}]}' is missing required fields '[ReferenceID, TotalGrossAmount]' @ 'CreateSupplierInvoice'"
        }
    ]
}