AutoMapper throwing NullReferenceException when mapping nullable enum to null

Question

I can't seem to figure out why AutoMapper is throwing an exception for this mapping:

public class MyDestinationType
{
    public MyCustomEnum? PropName { get; set; }
}

public enum MyCustomEnum
{
    Val1,
    Val2
}

Mapper.CreateMap<MySourceType, MyDestinationType>()
    .ForMember(d => d.PropName, o => o.ResolveUsing(s => 
    {
        if (s.Val1) return MyCustomEnum.Val1;
        else if (s.Val2) return MyCustomEnum.Val2;
        return null;
    }))
;

The exception only occurs when the MyCustomEnum? PropName property gets mapped to a null value. Here is how I am doing the mapping:

var source = MethodToGetSourceObject();
var destination = Mapper.Map<MyDestinationType>(source);

The exception thrown is an AutoMapperMappingException which wraps 2 other AutoMapperMappingExceptions. The 4th and final InnerException is a NullReferenceException. The stack trace looks something like this:

[NullReferenceException: Object reference not set to an instance of an object.]
   AutoMapper.Mappers.EnumMapper.Map(ResolutionContext context, IMappingEngineRunner mapper) +198
   AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.Map(ResolutionContext context) +447

[AutoMapperMappingException: Trying to map System.Object to System.Nullable`1[[MyProject.MyCustomEnum, MyProject, Version=2.0.4784.16259, Culture=neutral, PublicKeyToken=null]].
Using mapping configuration for MyProject.MySourceType to MyProject.MyDestinationType
Destination property: PropName
Exception of type 'AutoMapper.AutoMapperMappingException' was thrown.]
   AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.Map(ResolutionContext context) +519
   AutoMapper.Mappers.PropertyMapMappingStrategy.MapPropertyValue(ResolutionContext context, IMappingEngineRunner mapper, Object mappedObject, PropertyMap propertyMap) +592

[AutoMapperMappingException: Trying to map System.Object to System.Nullable`1[[MyProject.MyCustomEnum, MyProject, Version=2.0.4784.16259, Culture=neutral, PublicKeyToken=null]].
Using mapping configuration for MyProject.MySourceType to MyProject.MyDestinationType
Destination property: PropName
Exception of type 'AutoMapper.AutoMapperMappingException' was thrown.]
   AutoMapper.Mappers.PropertyMapMappingStrategy.MapPropertyValue(ResolutionContext context, IMappingEngineRunner mapper, Object mappedObject, PropertyMap propertyMap) +689
   AutoMapper.Mappers.PropertyMapMappingStrategy.Map(ResolutionContext context, IMappingEngineRunner mapper) +275
   AutoMapper.Mappers.TypeMapMapper.Map(ResolutionContext context, IMappingEngineRunner mapper) +273
   AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.Map(ResolutionContext context) +447

[AutoMapperMappingException: Trying to map MyProject.MySourceType to MyProject.MyDestinationType.
Using mapping configuration for MyProject.MySourceType to MyProject.MyDestinationType
Exception of type 'AutoMapper.AutoMapperMappingException' was thrown.]
   AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.Map(ResolutionContext context) +519
   AutoMapper.MappingEngine.Map(Object source, Type sourceType, Type destinationType, Action`1 opts) +192
   AutoMapper.MappingEngine.Map(Object source, Type sourceType, Type destinationType) +196
   AutoMapper.MappingEngine.Map(Object source) +169
   AutoMapper.Mapper.Map(Object source) +107
   [Here is where my code calls Mapper.Map<MyDestinationType>(source)]

Again, the exception only occurs when the resolution would have PropName == null (for example if both s.Val1 and s.Val2 were false). The exception does not occur when the mapping would resolve to the enum having some non-null value.

Is this another bug in AutoMapper?