diff --git a/management/univention-management-console-module-ucr/umc/python/ucr/__init__.py b/management/univention-management-console-module-ucr/umc/python/ucr/__init__.py
index 503b6f2..f6418d0 100644
--- a/management/univention-management-console-module-ucr/umc/python/ucr/__init__.py
+++ b/management/univention-management-console-module-ucr/umc/python/ucr/__init__.py
@@ -32,13 +32,14 @@
 # <http://www.gnu.org/licenses/>.
 
 from univention.lib.i18n import Translation
-from univention.management.console.modules import Base
+from univention.management.console.modules import Base, UMC_Error
+from univention.management.console.config import ucr
 from univention.management.console.protocol.definitions import BAD_REQUEST_INVALID_OPTS, SUCCESS
 
 from univention.management.console.modules.decorators import simple_response, sanitize
 from univention.management.console.modules.sanitizers import PatternSanitizer, ChoicesSanitizer
 
-import univention.config_registry as ucr
+from univention.config_registry import handler_set, handler_unset, ConfigRegistry, validate_key
 from univention.config_registry_info import ConfigRegistryInfo, Variable
 
 import univention.info_tools as uit
@@ -87,7 +88,12 @@ def is_readonly( self, key ):
 
 	def add( self, request ):
 		# does the same as put
-		self.put( request )
+		ucr.load()
+		already_set = set(ucr.keys()) & set(v['object']['key'] for v in request.options.values())
+		if already_set:
+			raise UMC_Error(_('The keys %r are already set.') % (', '.join(already_set))
+
+		self.put(request)
 
 	def put( self, request ):
 		message = ''
@@ -99,24 +105,19 @@ def put( self, request ):
 					var = _var['object']
 					value = var['value'] or ''
 					key = var['key']
+					if not validate_key(key):
+						raise UMC_Error(_('The UCR variable %r has an invalid name.') % (key,))
 					if self.is_readonly( key ):
-						success = False
-						message = _( 'The UCR variable %s is read-only and can not be changed!' ) % key
-						break
+						raise UMC_Error(_('The UCR variable %s is read-only and can not be changed!') % (key,))
 					arg = [ '%s=%s' % ( key.encode(), value.encode() ) ]
-					ucr.handler_set( arg )
+					handler_set( arg )
 
 					# handle descriptions, type, and categories
 					if 'descriptions' in var or 'type' in var or 'categories' in var:
 						self.__create_variable_info( var )
 				except KeyError:
 					# handle the case that neither key nor value are given for an UCR variable entry
-					request.status = BAD_REQUEST_INVALID_OPTS
-					self.finished(request.id, False, message = _('Invalid UCR variable entry, the properties "key" and "value" need to be specified.'))
-					return
-		else:
-			success = False
-			request.status = BAD_REQUEST_INVALID_OPTS
+					raise UMC_Error(_('Invalid UCR variable entry, the properties "key" and "value" need to be specified.')))
 
 		self.finished( request.id, success, message )
 
@@ -128,11 +129,11 @@ def remove( self, request ):
 				self.finished( request.id, False, message )
 				return
 
-		ucr.handler_unset( variables )
+		handler_unset( variables )
 		self.finished( request.id, True )
 
 	def get( self, request ):
-		ucrReg = ucr.ConfigRegistry()
+		ucrReg = ConfigRegistry()
 		ucrReg.load()
 		ucrInfo = ConfigRegistryInfo( registered_only = False )