when i build mapping builder using IMappingBuilderAdapter<>.Build() it throws error like:

System.ArgumentNullException: Value cannot be null.
Parameter name: key
   at System.Collections.Generic.Dictionary`2.FindEntry(TKey key)
   at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
   at Xtensive.ObjectMapping.MappingBuilder.BindToSource(TargetTypeDescription targetType, TargetPropertyDescription property) in c:\Work\DataObjects.Net\4.4\Xtensive.Core\Xtensive.Core\ObjectMapping\MappingBuilder.cs:line 251
   at Xtensive.ObjectMapping.MappingBuilder.BuildTargetPropertyDescriptions(TargetTypeDescription targetType) in c:\Work\DataObjects.Net\4.4\Xtensive.Core\Xtensive.Core\ObjectMapping\MappingBuilder.cs:line 239
   at Xtensive.ObjectMapping.MappingBuilder.BuildTargetDescriptions() in c:\Work\DataObjects.Net\4.4\Xtensive.Core\Xtensive.Core\ObjectMapping\MappingBuilder.cs:line 217
   at Xtensive.ObjectMapping.MappingBuilder.Build() in c:\Work\DataObjects.Net\4.4\Xtensive.Core\Xtensive.Core\ObjectMapping\MappingBuilder.cs:line 128

It is hard to find whats wrong with mapping. Later i found (thanks to access to DO4 repository :-)) that i have property "XYZ" in real entity class but does not have such property in DTO class and this is an reason why its fails. Could you add some check for missing mapping of properties and throw your own exception with detailed error which property missing on which type?

Update: v4.4 Build number 7589