diff --git a/src/objects.js b/src/objects.js
index 9047194..be0d70e 100644
--- a/src/objects.js
+++ b/src/objects.js
@@ -234,7 +234,7 @@ export function stringify (obj, options = {}) {
 		switch (obj) {
 			case undefined:
 			case null:
-				return obj;
+				return obj + "";
 		}
 
 		switch (typeof obj) {
diff --git a/tests/stringify.js b/tests/stringify.js
index 8c5e682..36be78e 100644
--- a/tests/stringify.js
+++ b/tests/stringify.js
@@ -3,6 +3,8 @@ import { stringify } from "../src/util.js";
 export default {
 	run: stringify,
 	tests: [
+		{ arg: undefined, expect: "undefined" },
+		{ arg: null, expect: "null" },
 		{ arg: 1, expect: "1" },
 		{ arg: NaN, expect: "NaN" },
 		{ arg: "foo", expect: '"foo"' },