diff --git a/management/univention-directory-manager-modules/modules/univention/admin/handlers/nagios/service.py b/management/univention-directory-manager-modules/modules/univention/admin/handlers/nagios/service.py
index 5acaf72..5f17f61 100644
--- a/management/univention-directory-manager-modules/modules/univention/admin/handlers/nagios/service.py
+++ b/management/univention-directory-manager-modules/modules/univention/admin/handlers/nagios/service.py
@@ -39,6 +39,7 @@
 import univention.admin.handlers
 import univention.admin.syntax
 import univention.admin.localization
+import univention.admin.uexceptions
 from univention.admin import configRegistry
 
 translation = univention.admin.localization.translation('univention.admin.handlers.nagios')
@@ -377,12 +378,16 @@ def _ldap_modlist(self):
 		if self.hasChanged('assignedHosts'):
 			hostlist = []
 			for hostdn in self.info.get('assignedHosts', []):
-				domain = self.lo.getAttr(hostdn, 'associatedDomain')
-				cn = self.lo.getAttr(hostdn, 'cn')
-				if not domain:
-					domain = [configRegistry.get("domainname")]
-				fqdn = "%s.%s" % (cn[0], domain[0])
-				hostlist.append(fqdn)
+				try:
+					host = self.lo.get(hostdn, ['associatedDomain', 'cn'], required=True)
+					cn = host['cn']
+				except univention.admin.uexceptions.noObject:
+					raise univention.admin.uexceptions.valueError('%r does not exists' % (hostdn,))
+				except KeyError:
+					raise univention.admin.uexceptions.valueError('%r has no cn attribute' % (hostdn,))
+
+				domain = host.get('associatedDomain', [configRegistry.get("domainname")])
+				hostlist.append("%s.%s" % (cn[0], domain[0]))
 
 			ml.insert(0, ('univentionNagiosHostname', self.oldattr.get('univentionNagiosHostname', []), hostlist))