diff --git a/lib/group/example.php b/lib/group/example.php
index f9c62fb97891e316545dff210494f9728f07674e..9c9ece5ac7758003b03b7946c1ce29d4d1fddfcf 100644
--- a/lib/group/example.php
+++ b/lib/group/example.php
@@ -98,7 +98,7 @@ abstract class OC_Group_Example {
 	 * @param string $gid
 	 * @return bool
 	 */
-	public function groupExists($gid);
+	abstract public function groupExists($gid);
 
 	/**
 	 * @brief get a list of all users in a group