diff --git a/src/model/encoding/DraftStringKey.js b/src/model/encoding/DraftStringKey.js
index a96bafd792..de454275a3 100644
--- a/src/model/encoding/DraftStringKey.js
+++ b/src/model/encoding/DraftStringKey.js
@@ -12,7 +12,7 @@
 'use strict';
 
 const DraftStringKey = {
-  stringify: function(key: mixed): string {
+  stringify: function(key: ?string): string {
     return '_' + String(key);
   },
 
diff --git a/src/model/encoding/__tests__/DraftStringKey-test.js b/src/model/encoding/__tests__/DraftStringKey-test.js
new file mode 100644
index 0000000000..8e37d99efc
--- /dev/null
+++ b/src/model/encoding/__tests__/DraftStringKey-test.js
@@ -0,0 +1,25 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails oncall+draft_js
+ * @flow strict-local
+ * @format
+ */
+
+'use strict';
+
+const {stringify, unstringify} = require('DraftStringKey');
+
+test('must convert maybe strings to a string key', () => {
+  expect(stringify('anything')).toEqual('_anything');
+  expect(stringify(null)).toEqual('_null');
+});
+
+test('must convert string keys back to a string', () => {
+  expect(unstringify('_anything')).toEqual('anything');
+  // This is a lossy conversion
+  expect(unstringify('_null')).toEqual('null');
+});