diff --git a/src/cache/local-storage-assignment-cache.spec.ts b/src/cache/local-storage-assignment-cache.spec.ts
index be5c58bc..9803ce94 100644
--- a/src/cache/local-storage-assignment-cache.spec.ts
+++ b/src/cache/local-storage-assignment-cache.spec.ts
@@ -5,6 +5,15 @@
 import { LocalStorageAssignmentCache } from './local-storage-assignment-cache';
 
 describe('LocalStorageAssignmentCache', () => {
+  beforeEach(() => {
+    window.localStorage.clear();
+  });
+
+  it('does not crash when localStorage contains a key named "undefined"', () => {
+    window.localStorage.setItem('undefined', 'not-json');
+    expect(() => new LocalStorageAssignmentCache('test')).not.toThrow();
+  });
+
   it('typical behavior', () => {
     const cache = new LocalStorageAssignmentCache('test');
     expect(
diff --git a/src/cache/local-storage-assignment-shim.ts b/src/cache/local-storage-assignment-shim.ts
index 57327b06..59332ce4 100644
--- a/src/cache/local-storage-assignment-shim.ts
+++ b/src/cache/local-storage-assignment-shim.ts
@@ -28,7 +28,9 @@ export class LocalStorageAssignmentShim implements Map<string, string> {
     this.getCache().forEach(callbackfn, thisArg);
   }
 
-  size: number = this.getCache().size;
+  get size(): number {
+    return this.getCache().size;
+  }
 
   entries(): IterableIterator<[string, string]> {
     return this.getCache().entries();
@@ -46,7 +48,7 @@ export class LocalStorageAssignmentShim implements Map<string, string> {
     return this.getCache()[Symbol.iterator]();
   }
 
-  [Symbol.toStringTag]: string = this.getCache()[Symbol.toStringTag];
+  [Symbol.toStringTag] = 'Map';
 
   public has(key: string): boolean {
     return this.getCache().has(key);