AppSync JavaScript Resolvers util.error Not Working

Hello,

I have just started using the javaScript AppSync resolvers after using VTL for years. One issue I am coming across is the util.error(...) or util.appendError(...) utility functions appear to have no effect.

My set-up currently includes a function to call and http data source, and a query which uses this function. My function calls an API Gateway REST api with a GET request. The response mapping template simply returns the parsed JSON, and if the response was not 200, call appendError.

import { util } from '@aws-appsync/utils';

export function request(ctx) {
    return {
        method: 'GET',
        params: {
            headers: {
                'Content-Type': 'application/json',
                'X-Api-Key': '<redacted>'
            }
        },
        resourcePath: '/dev/<redacted>/' + ctx.stash.id + '.json'
    };
}

export function response(ctx) {
    if (ctx.result.statusCode !== 200){
        return util.appendError(ctx.result.body, ctx.result.statusCode);
    }
    else {
        return JSON.parse(ctx.result.body);
    }
}

My query is as follows:

import { util } from '@aws-appsync/utils';


export function request(ctx) {

    ctx.stash.id = ctx.args.id

    return {};
}

export function response(ctx) {
    return ctx.result;
}

Part of the the ResponseFunctionEvaluation in CloudWatch is as follows:

"context": {
        "arguments": {
            "id": "testr"
        },
        "result": {
            "headers": {
                "content-length": "38",
                "Content-Type": "application/json",
                ...
                "X-Cache": "Error from cloudfront"
            },
            "statusCode": 404,
            "body": "{\"message\": \"That key does not exist\"}"
        },
        "prev": {
            "result": {}
        },
        "stash": {
            "id": "testr"
        },
        "outErrors": []
    },
"fieldInError": false,
"errors": [],

And part of the AfterResponseFunctionEvaluation:

"context": {
        "arguments": {
            "id": "testr"
        },
        "prev": {},
        "stash": {
            "id": "testr"
        },
        "outErrors": []
    },
"fieldInError": false,
"errors": [],

VTL resolvers appear to work correctly. I am not sure what I am doing wrong here?