diff --git a/functions/composer-storage-trigger/index.js b/functions/composer-storage-trigger/index.js
index 455837c880..84d223d924 100644
--- a/functions/composer-storage-trigger/index.js
+++ b/functions/composer-storage-trigger/index.js
@@ -79,6 +79,9 @@ function authorizeIap (clientId, projectId, userAgent) {
     })
     .then(res => res.json())
     .then(function obtainAccessTokenCallback (tokenResponse) {
+      if (tokenResponse.error) {
+        return Promise.reject(tokenResponse.error);
+      }
       var accessToken = tokenResponse.access_token;
       var iat = Math.floor(new Date().getTime() / 1000);
       var claims = {
@@ -104,6 +107,9 @@ function authorizeIap (clientId, projectId, userAgent) {
     })
     .then(res => res.json())
     .then(function signJsonClaimCallback (body) {
+      if (body.error) {
+        return Promise.reject(body.error);
+      }
       // Request service account signature on header and claimset
       var jwtSignature = body.signature;
       jwt = [JWT_HEADER, jwtClaimset, jwtSignature].join('.');
@@ -118,6 +124,9 @@ function authorizeIap (clientId, projectId, userAgent) {
     })
     .then(res => res.json())
     .then(function returnJwt (body) {
+      if (body.error) {
+        return Promise.reject(body.error);
+      }
       return {
         jwt: jwt,
         idToken: body.id_token
diff --git a/functions/composer-storage-trigger/package.json b/functions/composer-storage-trigger/package.json
index c2dcd939b3..5a8cf7569b 100644
--- a/functions/composer-storage-trigger/package.json
+++ b/functions/composer-storage-trigger/package.json
@@ -16,9 +16,14 @@
     "url": "https://github.com/GoogleCloudPlatform/nodejs-docs-samples.git"
   },
   "devDependencies": {
-    "@google-cloud/nodejs-repo-tools": "^2.2.5"
+    "@google-cloud/nodejs-repo-tools": "^2.2.5",
+    "ava": "0.25.0",
+    "proxyquire": "2.0.0",
+    "semistandard": "^12.0.1",
+    "sinon": "4.4.2"
   },
   "scripts": {
-    "lint": "repo-tools lint"
+    "lint": "repo-tools lint",
+    "test": "ava -T 20s --verbose test/*.test.js"
   }
 }
diff --git a/functions/composer-storage-trigger/test/index.test.js b/functions/composer-storage-trigger/test/index.test.js
new file mode 100644
index 0000000000..db4586d17c
--- /dev/null
+++ b/functions/composer-storage-trigger/test/index.test.js
@@ -0,0 +1,56 @@
+/**
+ * Copyright 2018 Google LLC
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+const proxyquire = require(`proxyquire`).noCallThru();
+const sinon = require(`sinon`);
+const test = require(`ava`);
+
+function getSample () {
+  const bodyJson = {};
+  const body = {
+    json: sinon.stub().resolves(bodyJson)
+  };
+  const FetchMock = sinon.stub().resolves(body);
+
+  return {
+    program: proxyquire(`../`, {
+      'node-fetch': FetchMock
+    }),
+    mocks: {
+      fetch: FetchMock,
+      body: body,
+      bodyJson: bodyJson
+    }
+  };
+}
+
+test.cb(`Handles error in JSON body`, (t) => {
+  const event = {
+    data: {
+      file: `some-file`
+    }
+  };
+  const expectedMsg = `Something bad happened.`;
+  const sample = getSample();
+  sample.mocks.bodyJson.error = expectedMsg;
+
+  sample.program.triggerDag(event, (err, message) => {
+    t.regex(err, /Something bad happened/);
+    t.is(message, undefined);
+    t.end();
+  });
+});