diff --git a/lib/util.php b/lib/util.php
index e8d4e56ef17..1fa3ad765d0 100755
--- a/lib/util.php
+++ b/lib/util.php
@@ -418,7 +418,8 @@ class OC_Util {
     public static function getInstanceId() {
         $id = OC_Config::getValue('instanceid', null);
         if(is_null($id)) {
-            $id = uniqid();
+            // We need to guarantee at least one letter in instanceid so it can be used as the session_name
+            $id = 'oc' . uniqid();
             OC_Config::setValue('instanceid', $id);
         }
         return $id;
diff --git a/tests/lib/util.php b/tests/lib/util.php
index 1c9054264c9..1f253825920 100644
--- a/tests/lib/util.php
+++ b/tests/lib/util.php
@@ -54,4 +54,9 @@ class Test_Util extends PHPUnit_Framework_TestCase {
 		$this->assertEquals('no-reply@example.com', $email);
 		OC_Config::deleteKey('mail_domain');
 	}
+
+  function testGetInstanceIdGeneratesValidId() {
+    OC_Config::deleteKey('instanceid');
+    $this->assertStringStartsWith('oc', OC_Util::getInstanceId());
+  }
 }