diff --git a/source/index.ts b/source/index.ts
index ffb2ad2..d9fb699 100644
--- a/source/index.ts
+++ b/source/index.ts
@@ -4,8 +4,6 @@
 
 import {Class, TypedArray, ObservableLike, Primitive} from './types';
 
-export {Class, TypedArray, ObservableLike, Primitive};
-
 const typedArrayTypeNames = [
 	'Int8Array',
 	'Uint8Array',
@@ -260,8 +258,7 @@ is.primitive = (value: unknown): value is Primitive => is.null_(value) || isPrim
 is.integer = (value: unknown): value is number => Number.isInteger(value as number);
 is.safeInteger = (value: unknown): value is number => Number.isSafeInteger(value as number);
 
-type ObjectKey = string | number | symbol;
-is.plainObject = <Value = unknown>(value: unknown): value is Record<ObjectKey, Value> => {
+is.plainObject = <Value = unknown>(value: unknown): value is Record<PropertyKey, Value> => {
 	// From: https://github.com/sindresorhus/is-plain-obj/blob/main/index.js
 	if (toString.call(value) !== '[object Object]') {
 		return false;
@@ -504,7 +501,7 @@ interface Assert {
 	primitive: (value: unknown) => asserts value is Primitive;
 	integer: (value: unknown) => asserts value is number;
 	safeInteger: (value: unknown) => asserts value is number;
-	plainObject: <Value = unknown>(value: unknown) => asserts value is Record<ObjectKey, Value>;
+	plainObject: <Value = unknown>(value: unknown) => asserts value is Record<PropertyKey, Value>;
 	typedArray: (value: unknown) => asserts value is TypedArray;
 	arrayLike: <T = unknown>(value: unknown) => asserts value is ArrayLike<T>;
 	domElement: (value: unknown) => asserts value is HTMLElement;
@@ -601,7 +598,7 @@ export const assert: Assert = {
 	primitive: (value: unknown): asserts value is Primitive => assertType(is.primitive(value), AssertionTypeDescription.primitive, value),
 	integer: (value: unknown): asserts value is number => assertType(is.integer(value), AssertionTypeDescription.integer, value),
 	safeInteger: (value: unknown): asserts value is number => assertType(is.safeInteger(value), AssertionTypeDescription.safeInteger, value),
-	plainObject: <Value = unknown>(value: unknown): asserts value is Record<ObjectKey, Value> => assertType(is.plainObject(value), AssertionTypeDescription.plainObject, value),
+	plainObject: <Value = unknown>(value: unknown): asserts value is Record<PropertyKey, Value> => assertType(is.plainObject(value), AssertionTypeDescription.plainObject, value),
 	typedArray: (value: unknown): asserts value is TypedArray => assertType(is.typedArray(value), AssertionTypeDescription.typedArray, value),
 	arrayLike: <T = unknown>(value: unknown): asserts value is ArrayLike<T> => assertType(is.arrayLike(value), AssertionTypeDescription.arrayLike, value),
 	domElement: (value: unknown): asserts value is HTMLElement => assertType(is.domElement(value), AssertionTypeDescription.domElement, value),
@@ -662,6 +659,8 @@ Object.defineProperties(assert, {
 
 export default is;
 
+export {Class, TypedArray, ObservableLike, Primitive} from './types';
+
 // For CommonJS default export support
 module.exports = is;
 module.exports.default = is;