diff --git a/bind.js b/bind.js
index eaa1eaa..e3a189a 100644
--- a/bind.js
+++ b/bind.js
@@ -15,7 +15,8 @@ export default function classNames () {
 
 function parseValue (arg) {
 	if (typeof arg === 'string') {
-		return this && this[arg] || arg;
+		const mappedClass = this && this[arg];
+		return typeof mappedClass === 'string' && mappedClass || arg;
 	}
 
 	if (typeof arg !== 'object') {
@@ -34,7 +35,8 @@ function parseValue (arg) {
 
 	for (const key in arg) {
 		if (hasOwn.call(arg, key) && arg[key]) {
-			classes = appendClass(classes, this && this[key] || key);
+			const mappedClass = this && this[key];
+			classes = appendClass(classes, typeof mappedClass === 'string' && mappedClass || key);
 		}
 	}
 
diff --git a/tests/bind.js b/tests/bind.js
index ba375e6..b26156e 100644
--- a/tests/bind.js
+++ b/tests/bind.js
@@ -79,6 +79,31 @@ describe('bind', () => {
 	});
 
 	describe('classNamesBound', () => {
+		it('keeps unmapped string classes that match Object.prototype properties', () => {
+			for (const name of ['toString', 'constructor', 'hasOwnProperty', '__proto__']) {
+				assert.equal(classNamesBound(name), name);
+				assert.equal(classNamesBound([name]), name);
+				assert.equal(classNamesBound('a', name), '#a ' + name);
+			}
+		});
+
+		it('keeps unmapped object keys that match Object.prototype properties', () => {
+			for (const name of ['constructor', 'hasOwnProperty', '__proto__']) {
+				assert.equal(classNamesBound({[name]: true}), name);
+			}
+		});
+
+		it('uses string mappings for prototype-named and inherited classes', () => {
+			const mappings = Object.setPrototypeOf({
+				toString: '#toString', constructor: '#constructor', ['__proto__']: '#proto'
+			}, {inherited: '#inherited'});
+			const bound = classNames.bind(mappings);
+			assert.equal(bound('toString', 'constructor', '__proto__', 'inherited'),
+				'#toString #constructor #proto #inherited');
+			assert.equal(bound({constructor: true, ['__proto__']: true, inherited: true}),
+				'#constructor #proto #inherited');
+		});
+
 		it('keeps object keys with truthy values', () => {
 			assert.equal(classNamesBound({
 				a: true,