diff --git a/.eslintrc.yml b/.eslintrc.yml
index 21c50fcb..d31b2ae2 100644
--- a/.eslintrc.yml
+++ b/.eslintrc.yml
@@ -51,7 +51,7 @@ rules:
   node/no-unpublished-import: error
   node/no-unpublished-require: error
   node/no-unsupported-features/es-builtins: error
-  node/no-unsupported-features/es-syntax: off # TODO enable
+  node/no-unsupported-features/es-syntax: error
   node/no-unsupported-features/node-builtins: error
   node/process-exit-as-throw: error
   node/shebang: error
@@ -444,6 +444,8 @@ overrides:
     extends:
       - plugin:import/typescript
     rules:
+      node/no-unsupported-features/es-syntax: off
+
       ##########################################################################
       # `@typescript-eslint/eslint-plugin` rule list based on `v3.5.x`
       ##########################################################################
diff --git a/src/index.ts b/src/index.ts
index 47d232e1..23b4d930 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -15,7 +15,7 @@ import type {
   GraphQLFormattedError,
 } from 'graphql';
 import accepts from 'accepts';
-import httpError, { HttpError } from 'http-errors';
+import httpError from 'http-errors';
 import {
   Source,
   parse,
@@ -31,7 +31,9 @@ import type { GraphiQLOptions, GraphiQLData } from './renderGraphiQL';
 import { parseBody } from './parseBody';
 import { renderGraphiQL } from './renderGraphiQL';
 
-type Request = IncomingMessage;
+// `url` is always defined for IncomingMessage coming from http.Server
+type Request = IncomingMessage & { url: string };
+
 type Response = ServerResponse & { json?: (data: unknown) => void };
 type MaybePromise<T> = Promise<T> | T;
 
@@ -361,11 +363,10 @@ export function graphqlHTTP(options: Options): Middleware {
       // If an error was caught, report the httpError status, or 500.
       response.statusCode = error.status ?? 500;
 
-      if (error.headers) {
-        const err = error as HttpError;
-        // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-        for (const [key, value] of Object.entries(err.headers!)) {
-          response.setHeader(key, value);
+      const { headers } = error;
+      if (headers != null) {
+        for (const [key, value] of Object.entries(headers)) {
+          response.setHeader(key, String(value));
         }
       }
 
@@ -465,9 +466,7 @@ export interface GraphQLParams {
 export async function getGraphQLParams(
   request: Request,
 ): Promise<GraphQLParams> {
-  // `url` is always defined for IncomingMessage coming from http.Server
-  // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-  const urlData = new URLSearchParams(request.url!.split('?')[1]);
+  const urlData = new URLSearchParams(request.url.split('?')[1]);
   const bodyData = await parseBody(request);
 
   // GraphQL Query string.
diff --git a/src/parseBody.ts b/src/parseBody.ts
index adbf63b4..32b1f968 100644
--- a/src/parseBody.ts
+++ b/src/parseBody.ts
@@ -6,12 +6,9 @@ import querystring from 'querystring';
 import getBody from 'raw-body';
 import httpError from 'http-errors';
 import contentType from 'content-type';
+import type { ParsedMediaType } from 'content-type';
 
 type Request = IncomingMessage & { body?: unknown };
-interface ParsedMediaType {
-  type: string;
-  parameters: { [key: string]: string | undefined };
-}
 
 /**
  * Provided a "Request" provided by express or connect (typically a node style
@@ -106,7 +103,7 @@ async function readBody(
   } catch (err) {
     throw err.type === 'encoding.unsupported'
       ? httpError(415, `Unsupported charset "${charset.toUpperCase()}".`)
-      : httpError(400, `Invalid body: ${err.message as string}.`);
+      : httpError(400, `Invalid body: ${String(err.message)}.`);
   }
 }