diff --git a/src/ServiceAccountCredentials.php b/src/ServiceAccountCredentials.php
index 2e8810e68..e9a52a7e6 100644
--- a/src/ServiceAccountCredentials.php
+++ b/src/ServiceAccountCredentials.php
@@ -98,6 +98,10 @@ public function __construct($scope, $jsonKey,
   */
   public function getCacheKey()
   {
-    return $this->auth->getIssuer() . ':' . $this->auth->getCacheKey();
+    $key = $this->auth->getIssuer() . ':' . $this->auth->getCacheKey();
+    if ($sub = $this->auth->getSub()) {
+      $key .= ':' . $sub;
+    }
+    return $key;
   }
 }
diff --git a/tests/ServiceAccountCredentialsTest.php b/tests/ServiceAccountCredentialsTest.php
index 398a252aa..0c8ceb0ca 100644
--- a/tests/ServiceAccountCredentialsTest.php
+++ b/tests/ServiceAccountCredentialsTest.php
@@ -53,6 +53,23 @@ public function testShouldBeTheSameAsOAuth2WithTheSameScope()
         $sa->getCacheKey()
     );
   }
+
+  public function testShouldBeTheSameAsOAuth2WithTheSameScopeWithSub()
+  {
+    $testJson = createTestJson();
+    $scope = ['scope/1', 'scope/2'];
+    $sub = 'sub123';
+    $sa = new ServiceAccountCredentials(
+        $scope,
+        $testJson,
+        null,
+        $sub);
+    $o = new OAuth2(['scope' => $scope]);
+    $this->assertSame(
+        $testJson['client_email'] . ':' . $o->getCacheKey() . ':' . $sub,
+        $sa->getCacheKey()
+    );
+  }
 }
 
 class SACConstructorTest extends \PHPUnit_Framework_TestCase