diff --git a/src/tag.test.ts b/src/tag.test.ts
new file mode 100644
index 0000000..f21998c
--- /dev/null
+++ b/src/tag.test.ts
@@ -0,0 +1,17 @@
+import Tag from './tag';
+
+describe('Tag.isTag', function () {
+  it('should detect when value is not a tag', () => {
+    expect(Tag.isTag(undefined)).toBe(false);
+    expect(Tag.isTag(null)).toBe(false);
+    expect(Tag.isTag('')).toBe(false);
+    expect(Tag.isTag(8)).toBe(false);
+    expect(Tag.isTag(true)).toBe(false);
+    expect(Tag.isTag([])).toBe(false);
+    expect(Tag.isTag({ my: 'object' })).toBe(false);
+  });
+
+  it('should detect tags', () => {
+    expect(Tag.isTag(new Tag('tag', {}, []))).toBe(true);
+  });
+});
diff --git a/src/tag.ts b/src/tag.ts
index 1440d84..3d027bf 100644
--- a/src/tag.ts
+++ b/src/tag.ts
@@ -7,7 +7,7 @@ export default class Tag<
   readonly $$mdtype = 'Tag' as const;
 
   static isTag = (tag: any): tag is Tag => {
-    return '$$mdtype' in tag && tag.$$mdtype === 'Tag';
+    return !!(tag?.$$mdtype === 'Tag');
   };
 
   name: N;