diff --git a/src/Response.ts b/src/Response.ts
index 627747e..27addf4 100644
--- a/src/Response.ts
+++ b/src/Response.ts
@@ -1,12 +1,11 @@
 import _ from 'underscore';
 import cookie from 'cookie';
 import { Application, Request } from '.';
-import { StringMap, isStringMap, StringArrayOfStringsMap } from '@silvermine/toolbox';
+import { isUndefined, StringMap, isStringMap, StringArrayOfStringsMap } from '@silvermine/toolbox';
 import { CookieOpts, ResponseResult } from './request-response-types';
 import { StatusCodes } from './status-codes';
 import { Callback } from 'aws-lambda';
 import mimeLookup from './mime/mimeLookup';
-import { isUndefined } from 'util';
 
 export default class Response {
 
diff --git a/src/interfaces.ts b/src/interfaces.ts
index 807f41b..0b319fd 100644
--- a/src/interfaces.ts
+++ b/src/interfaces.ts
@@ -3,7 +3,6 @@
 import Request from './Request';
 import Response from './Response';
 import { LogLevel } from './logging/logging-types';
-import { isError } from 'util';
 import { isNumber } from '@silvermine/toolbox';
 import { StatusCodes } from './status-codes';
 
@@ -267,7 +266,7 @@ export interface RouteProcessorAppender<T> {
 export type ErrorWithStatusCode<T extends Error> = T & { statusCode?: number };
 
 export function isErrorWithStatusCode<T extends Error>(o: any): o is ErrorWithStatusCode<T> {
-   return isError(o)
+   return o instanceof Error
       && isNumber((o as any).statusCode)
       && StatusCodes[(o as any).statusCode] !== undefined;
 }