diff --git a/apps/files_external/lib/config.php b/apps/files_external/lib/config.php
index 94dc5fb7ad..cd3e7f3a4a 100755
--- a/apps/files_external/lib/config.php
+++ b/apps/files_external/lib/config.php
@@ -277,15 +277,21 @@ class OC_Mount_Config {
 										 $mountType,
 										 $applicable,
 										 $isPersonal = false) {
+		$backends = self::getBackends();
 		$mountPoint = OC\Files\Filesystem::normalizePath($mountPoint);
 		if ($mountPoint === '' || $mountPoint === '/' || $mountPoint == '/Shared') {
 			// can't mount at root or "Shared" folder
 			return false;
 		}
+
+		if (!isset($backends[$class])) {
+			// invalid backend
+			return false;
+		}	
 		if ($isPersonal) {
 			// Verify that the mount point applies for the current user
 			// Prevent non-admin users from mounting local storage
-			if ($applicable != OCP\User::getUser() || $class == '\OC\Files\Storage\Local') {
+			if ($applicable !== OCP\User::getUser() || strtolower($class) === '\oc\files\storage\local') {
 				return false;
 			}
 			$mountPoint = '/'.$applicable.'/files/'.ltrim($mountPoint, '/');
diff --git a/apps/files_external/tests/mountconfig.php b/apps/files_external/tests/mountconfig.php
index 941aec680b..24ebcf5134 100644
--- a/apps/files_external/tests/mountconfig.php
+++ b/apps/files_external/tests/mountconfig.php
@@ -48,4 +48,29 @@ class Test_Mount_Config extends \PHPUnit_Framework_TestCase {
 		$this->assertEquals(false, OC_Mount_Config::addMountPoint('/Shared', $storageClass, array(), $mountType, $applicable, $isPersonal));
 
 	}
+
+	public function testAddMountPointSingleUser() {
+		\OC_User::setUserId('test');
+		$mountType = 'user';
+		$applicable = 'test';
+		$isPersonal = true;
+		// local
+		$this->assertEquals(false, OC_Mount_Config::addMountPoint('/ext', '\OC\Files\storage\local', array(), $mountType, $applicable, $isPersonal));
+		// non-local
+		// FIXME: can't test this yet as the class (write operation) is not mockable
+		// $this->assertEquals(true, OC_Mount_Config::addMountPoint('/ext', '\OC\Files\Storage\SFTP', array(), $mountType, $applicable, $isPersonal));
+
+	}
+
+	public function testAddMountPointUnexistClass() {
+		\OC_User::setUserId('test');
+		$storageClass = 'Unexist_Storage';
+		$mountType = 'user';
+		$applicable = 'test';
+		$isPersonal = true;
+		// local
+		// non-local
+		$this->assertEquals(false, OC_Mount_Config::addMountPoint('/ext', $storageClass, array(), $mountType, $applicable, $isPersonal));
+
+	}
 }