diff --git a/management/univention-directory-manager-modules/modules/univention/admin/handlers/__init__.py b/management/univention-directory-manager-modules/modules/univention/admin/handlers/__init__.py
index cd54420..6a40599 100644
--- a/management/univention-directory-manager-modules/modules/univention/admin/handlers/__init__.py
+++ b/management/univention-directory-manager-modules/modules/univention/admin/handlers/__init__.py
@@ -106,6 +106,9 @@ def __init__(self, co, lo, position, dn='', superordinate = None ):
 				self.position.setDn(dn)
 		self._open = 0
 
+		if not isinstance(lo, univention.admin.uldap.access):
+			raise TypeError('lo must be instance of univention.admin.uldap.access.')
+
 	def open(self):
 		self._open = 1