diff --git a/packages/polyfill/source/Node.ts b/packages/polyfill/source/Node.ts
index af1a5705..864f4922 100644
--- a/packages/polyfill/source/Node.ts
+++ b/packages/polyfill/source/Node.ts
@@ -160,7 +160,7 @@ export class Node extends EventTarget {
     while (true) {
       if (currentNode == null) return false;
       if (currentNode === this) return true;
-      currentNode = node!.parentNode;
+      currentNode = currentNode.parentNode;
     }
   }
 }
diff --git a/packages/polyfill/source/tests/node.test.ts b/packages/polyfill/source/tests/node.test.ts
new file mode 100644
index 00000000..b557dcac
--- /dev/null
+++ b/packages/polyfill/source/tests/node.test.ts
@@ -0,0 +1,60 @@
+import {Window} from '../index.ts';
+
+import {describe, it, expect, beforeEach} from 'vitest';
+
+describe('Node#contains', () => {
+  beforeEach(() => {
+    const window = new Window();
+    Window.setGlobalThis(window);
+  });
+
+  it('returns true for a direct child', () => {
+    const parent = document.createElement('div');
+    const child = document.createElement('span');
+    parent.append(child);
+
+    expect(parent.contains(child)).toBe(true);
+  });
+
+  it('returns true for an indirect descendant', () => {
+    const ancestor = document.createElement('div');
+    const middle = document.createElement('div');
+    const descendant = document.createElement('span');
+    middle.append(descendant);
+    ancestor.append(middle);
+
+    // The walk used to re-read the original node's parentNode instead of
+    // advancing, so any indirect descendant looped forever.
+    expect(ancestor.contains(descendant)).toBe(true);
+  });
+
+  it('returns true for the node itself', () => {
+    const element = document.createElement('div');
+
+    expect(element.contains(element)).toBe(true);
+  });
+
+  it('returns false for a detached node', () => {
+    const element = document.createElement('div');
+    const detached = document.createElement('span');
+
+    expect(element.contains(detached)).toBe(false);
+  });
+
+  it('returns false for sibling subtrees', () => {
+    const root = document.createElement('div');
+    const left = document.createElement('div');
+    const right = document.createElement('div');
+    const rightChild = document.createElement('span');
+    right.append(rightChild);
+    root.append(left, right);
+
+    expect(left.contains(rightChild)).toBe(false);
+  });
+
+  it('returns false for null', () => {
+    const element = document.createElement('div');
+
+    expect(element.contains(null)).toBe(false);
+  });
+});