IList Property stays null even when member is instantiated

IList Property stays null even when member is instantiated

I am having trouble using an IList property which always seems to return
null, even though the member is is getting is instantiated:
private List<ModelRootEntity> _validTargets = new
List<ModelRootEntity>();
public IList<IModelRootEntity> ValidTargets
{
get
{
return _validTargets as IList<IModelRootEntity>;
}
protected internal set
{
if (value == null)
_validTargets.Clear();
else
_validTargets = value as List<ModelRootEntity>;
}
}
ModelRootEntity implements IModelRootEntity. I watched both values during
debugging, whilst the member shows a positive count, the property stays
null.
I also tried raising an exception within the property getter to throw if
the counts of _validTargets and _validTargets as List<ModelRootEntity> are
different, but it never threw.
Found question [Dictionary properties are always null despite dictionaries
being instantiated, which seems similar, however in my case this seems to
happen regardless of serialization.
Any ideas?