Shipping CODE Discount Function - Checkout Error

[bkrishnan]

I'm working on creating a Shopify function for creating custom shipping discount codes (Not automatic discounts).

The function gets invoked at checkout correctly when the code is applied
I'm able to see the expected input and output for the function in Shopify Partner's "Run Detail" section for the function. There are no errors there
But I get the following error on the checkout screen as soon as the code is applied:

There was a problem with our checkout
Refresh this page or try again in a few minutes

I followed the standard tutorial to build the discount function but modified it for creating a shipping discount instead: https://shopify.dev/docs/apps/selling-strategies/discounts/experience

Here's the mutation I used to create the discount without any error:

mutation {
  discountCodeAppCreate(codeAppDiscount: {
    code: "SHIPTEST",
    title: "Shipping Test",
    functionId: "01H4XPYSZTB3ANV48G0ZEH760M",
    startsAt: "2022-06-22T00:00:00"
  }) {
     codeAppDiscount {
      discountId
     }
     userErrors {
      field
      message
     }
  }
}

Here is the input for the function that gets invoked successfully at checkout:

{
  "cart": {
    "lines": [
      {
        "quantity": 3,
        "merchandise": {
          "__typename": "ProductVariant",
          "id": "gid://shopify/ProductVariant/45565892690203"
        }
      }
    ]
  }
}

Here is the output for the function that gets invoked successfully at checkout

{
  "discountApplicationStrategy": "FIRST",
  "discounts": [
    {
      "value": {
        "percentage": {
          "value": 100.0
        }
      },
      "targets": [
        {
          "deliveryGroup": {
            "id": "gid://shopify/CartDeliveryGroup/0"
          }
        }
      ],
      "message": "100% off"
    }
  ]
}

Now because I see no errors in my dev console, partner's dashboard etc., I have no way to figure out where/what the issue is.

I'm also including the graphql error as seen in the browser:

[
    {
        "message": "Internal error. Looks like something went wrong on our end.\nRequest ID: dab38efe-0531-4b2b-8378-6a88b63db1da (include this in support requests).",
        "extensions": {
            "code": "INTERNAL_SERVER_ERROR",
            "requestId": "dab38efe-0531-4b2b-8378-6a88b63db1da"
        }
    }
]