Modifying Total Cart Cost

[cherriecoder]

I am using the checkout extensibility function and am trying to update the total tax amount based on the customer name and shipping address (this is for tax exempt purposes) but I am running into some trouble and am not sure what's wrong. I am getting an error message 

[ { "path": [ "errors" ], "explanation": "Expected value to not be null" }, { "path": [ "updatedTaxAmount" ], "explanation": "Field is not defined on FunctionRunResult" } ]

Here is how I set up my file (I am trying to modify the cart cost price no matter the condition for now just to see if it will work)

import type {CartCost, FunctionError, FunctionRunResult, RunInput,} from "../generated/api";
import {CurrencyCode} from "../generated/api";

export function run(input: RunInput): FunctionRunResult {
  const currentBuyerAddress:string = input.cart.deliveryGroups[0]?.deliveryAddress?.address1 ?? '';
  let errors: FunctionError[] = input.cart.lines
    .filter(({ quantity }) => quantity > 1)
    .map(() => ({
      localizedMessage: "Not possible to order more than one of each",
      target: "cart",
    }));

  const updatedTaxAmount: CartCost = {
      subtotalAmount: {
        amount: "749.00",
        currencyCode: CurrencyCode.Usd
      },
      totalAmount:{
        amount: "749.00",
        currencyCode: CurrencyCode.Usd
      },
      totalTaxAmount: {
        amount: "0.00",
        currencyCode: CurrencyCode.Usd
      }
    };

  return {
    errors,
    updatedTaxAmount
  }
};

and here is what i added in my schema 

type FunctionRunResult {
  errors: [FunctionError!]!
  updatedTaxAmount: CartCostInput!
}


"""
The result of a cart validation function.
"""
input FunctionRunResult {
  """
  Errors.
  """
  errors: [FunctionError!]!

  """
  Updated Tax Amount.
  """
  updatedTaxAmount: CartCostInput!
}

input CartCostInput {
  subtotalAmount: MoneyV2Input!
  totalAmount: MoneyV2Input!
  totalTaxAmount: MoneyV2Input
}

input MoneyV2Input {
  amount: String!
  currencyCode: String!
}