diff --git a/.changeset/early-dots-peel.md b/.changeset/early-dots-peel.md
new file mode 100644
index 00000000000..2b059fbabff
--- /dev/null
+++ b/.changeset/early-dots-peel.md
@@ -0,0 +1,5 @@
+---
+"@firebase/rules-unit-testing": patch
+---
+
+Fix assertFails not recognising database permission denied error
diff --git a/packages/rules-unit-testing/src/api/index.ts b/packages/rules-unit-testing/src/api/index.ts
index 680b195b09f..28626127f52 100644
--- a/packages/rules-unit-testing/src/api/index.ts
+++ b/packages/rules-unit-testing/src/api/index.ts
@@ -454,9 +454,14 @@ export function assertFails(pr: Promise<any>): any {
       );
     },
     (err: any) => {
+      const errCode = (err && err.code && err.code.toLowerCase()) || '';
+      const errMessage =
+        (err && err.message && err.message.toLowerCase()) || '';
       const isPermissionDenied =
-        (err && err.message && err.message.indexOf('PERMISSION_DENIED') >= 0) ||
-        (err && err.code === 'permission-denied');
+        errCode === 'permission-denied' ||
+        errCode === 'permission_denied' ||
+        errMessage.indexOf('permission_denied') >= 0;
+
       if (!isPermissionDenied) {
         return Promise.reject(
           new Error(
diff --git a/packages/rules-unit-testing/test/database.test.ts b/packages/rules-unit-testing/test/database.test.ts
index d5929ee8666..95bb16a8db7 100644
--- a/packages/rules-unit-testing/test/database.test.ts
+++ b/packages/rules-unit-testing/test/database.test.ts
@@ -103,6 +103,31 @@ describe('Testing Module Tests', function () {
       .catch(() => {});
   });
 
+  it('assertFails() if code is PERMISSION_DENIED', async function () {
+    const success = Promise.resolve('success');
+    const permissionDenied = Promise.reject({
+      code: 'PERMISSION_DENIED'
+    });
+    const otherFailure = Promise.reject('failure');
+    await firebase
+      .assertFails(success)
+      .then(() => {
+        throw new Error('Expected success to fail.');
+      })
+      .catch(() => {});
+
+    await firebase.assertFails(permissionDenied).catch(() => {
+      throw new Error('Expected permissionDenied to succeed.');
+    });
+
+    await firebase
+      .assertFails(otherFailure)
+      .then(() => {
+        throw new Error('Expected otherFailure to fail.');
+      })
+      .catch(() => {});
+  });
+
   it('initializeTestApp() with auth=null does not set access token', async function () {
     const app = firebase.initializeTestApp({
       projectId: 'foo',