diff --git a/package.json b/package.json
index 6d6a16e..9695716 100644
--- a/package.json
+++ b/package.json
@@ -43,6 +43,8 @@
 		"call-bind": "^1.0.8",
 		"define-properties": "^1.2.1",
 		"es-errors": "^1.3.0",
+		"get-proto": "^1.0.1",
+		"gopd": "^1.2.0",
 		"set-function-name": "^2.0.2"
 	},
 	"devDependencies": {
diff --git a/shim.js b/shim.js
index 3ec77c1..9d22c8b 100644
--- a/shim.js
+++ b/shim.js
@@ -2,15 +2,15 @@
 
 var supportsDescriptors = require('define-properties').supportsDescriptors;
 var getPolyfill = require('./polyfill');
-var gOPD = Object.getOwnPropertyDescriptor;
+var gOPD = require('gopd');
 var defineProperty = Object.defineProperty;
-var TypeErr = TypeError;
-var getProto = Object.getPrototypeOf;
+var $TypeError = require('es-errors');
+var getProto = require('get-proto');
 var regex = /a/;
 
 module.exports = function shimFlags() {
 	if (!supportsDescriptors || !getProto) {
-		throw new TypeErr('RegExp.prototype.flags requires a true ES5 environment that supports property descriptors');
+		throw new $TypeError('RegExp.prototype.flags requires a true ES5 environment that supports property descriptors');
 	}
 	var polyfill = getPolyfill();
 	var proto = getProto(regex);