diff --git a/js/.changeset/fix-normalize-selector-array-validation.md b/js/.changeset/fix-normalize-selector-array-validation.md
new file mode 100644
index 0000000..3b42ad9
--- /dev/null
+++ b/js/.changeset/fix-normalize-selector-array-validation.md
@@ -0,0 +1,11 @@
+---
+'browser-commander': patch
+---
+
+Fix normalizeSelector to validate input type and reject arrays
+
+When `normalizeSelector` receives an invalid type (array, number, or non-text-selector object), it now returns `null` with a warning instead of returning the invalid value unchanged.
+
+This prevents downstream `querySelectorAll` errors with invalid selector syntax (like trailing commas when arrays are accidentally passed).
+
+Fixes #23
diff --git a/js/src/elements/selectors.js b/js/src/elements/selectors.js
index b5b6dbc..0b00d1b 100644
--- a/js/src/elements/selectors.js
+++ b/js/src/elements/selectors.js
@@ -162,6 +162,25 @@ export async function normalizeSelector(options = {}) {
     throw new Error('selector is required in options');
   }
 
+  // DEFENSIVE: Validate selector type - must be string or Puppeteer text selector object
+  // Arrays and other invalid types should be rejected to prevent downstream querySelectorAll errors
+  if (Array.isArray(selector)) {
+    console.warn(
+      `normalizeSelector received invalid selector type: array. Expected string or text selector object.`
+    );
+    return null;
+  }
+
+  if (
+    typeof selector !== 'string' &&
+    (typeof selector !== 'object' || !selector._isPuppeteerTextSelector)
+  ) {
+    console.warn(
+      `normalizeSelector received invalid selector type: ${typeof selector}. Expected string or text selector object.`
+    );
+    return null;
+  }
+
   // Handle Playwright text selectors (strings containing :has-text or :text-is)
   // These are valid for Playwright's locator API but NOT for document.querySelectorAll
   if (
@@ -267,6 +286,8 @@ export async function normalizeSelector(options = {}) {
     }
   }
 
+  // This line should be unreachable after validation, but kept as a safeguard
+  // istanbul ignore next
   return selector;
 }
 
diff --git a/js/tests/unit/elements/selectors.test.js b/js/tests/unit/elements/selectors.test.js
index f92fc97..b1c848f 100644
--- a/js/tests/unit/elements/selectors.test.js
+++ b/js/tests/unit/elements/selectors.test.js
@@ -207,11 +207,41 @@ describe('selectors', () => {
       assert.strictEqual(result, 'button');
     });
 
-    it('should return non-text-selector object unchanged', async () => {
+    it('should return null for invalid object selector', async () => {
       const page = createMockPlaywrightPage();
       const obj = { someKey: 'value' };
       const result = await normalizeSelector({ page, selector: obj });
-      assert.strictEqual(result, obj);
+      assert.strictEqual(result, null);
+    });
+
+    it('should return null for array selector', async () => {
+      const page = createMockPlaywrightPage();
+      const arr = ['[data-qa="test"]', []];
+      const result = await normalizeSelector({ page, selector: arr });
+      assert.strictEqual(result, null);
+    });
+
+    it('should return null for number selector', async () => {
+      const page = createMockPlaywrightPage();
+      const result = await normalizeSelector({ page, selector: 123 });
+      assert.strictEqual(result, null);
+    });
+
+    it('should accept valid Puppeteer text selector object', async () => {
+      const page = createMockPuppeteerPage();
+      page.evaluate = async () => '[data-qa="test"]';
+      const textSelector = {
+        _isPuppeteerTextSelector: true,
+        baseSelector: 'button',
+        text: 'Click me',
+        exact: false,
+      };
+      const result = await normalizeSelector({
+        page,
+        engine: 'puppeteer',
+        selector: textSelector,
+      });
+      assert.strictEqual(result, '[data-qa="test"]');
     });
   });