--- a/app/models/custom_field.rb
+++ b/app/models/custom_field.rb
@@ -121,11 +121,12 @@ class CustomField < ActiveRecord::Base
   # objects by their value of the custom field.
   # Returns false, if the custom field can not be used for sorting.
   def order_statement
+    customized_type_classname = (self.class.customized_class.name == 'User') ? self.class.customized_class.superclass.name : self.class.customized_class.name
     case field_format
       when 'string', 'text', 'list', 'date', 'bool'
         # COALESCE is here to make sure that blank and NULL values are sorted equally
         "COALESCE((SELECT cv_sort.value FROM #{CustomValue.table_name} cv_sort" +
-          " WHERE cv_sort.customized_type='#{self.class.customized_class.name}'" +
+          " WHERE cv_sort.customized_type='#{customized_type_classname}'" +
           " AND cv_sort.customized_id=#{self.class.customized_class.table_name}.id" +
           " AND cv_sort.custom_field_id=#{id} LIMIT 1), '')"
       when 'int', 'float'
@@ -133,7 +134,7 @@ class CustomField < ActiveRecord::Base
         # Postgresql will raise an error if a value can not be casted!
         # CustomValue validations should ensure that it doesn't occur
         "(SELECT CAST(cv_sort.value AS decimal(60,3)) FROM #{CustomValue.table_name} cv_sort" +
-          " WHERE cv_sort.customized_type='#{self.class.customized_class.name}'" +
+          " WHERE cv_sort.customized_type='#{customized_type_classname}'" +
           " AND cv_sort.customized_id=#{self.class.customized_class.table_name}.id" +
           " AND cv_sort.custom_field_id=#{id} AND cv_sort.value <> '' AND cv_sort.value IS NOT NULL LIMIT 1)"
       else