C# LINQ query fails when using IsAbstract property during derived class scan with reflection?

I am using reflection to scan for all the class types that derive (are assignable) from a particular base class. That part works fine. But when I try to filter the resulting list by the IsAbstract property to get a list of only the non-abstract classes, the LINQ query that does the filtering fails to work correctly. Instead, I had to resort to a foreach loop and do it "manually." I tried this query first:

       if (!bAcceptAbstract)
           retListFiltered = (from typeClass in retList where typeClass.GetType().IsAbstract == false select typeClass).ToList();

But that didn't filter out the class types marked as abstract.

I then tried:

retListFiltered = (from typeClass in retList where !typeClass.GetType().IsAbstract select typeClass).ToList();

But again, no filtering. Why doesn't the LINQ query seem to respect the value of the IsAbstract propery?

Below is the code I ended up having to use:

   public static List<System.Type> GetAllDerivedTypes<T>(Assembly primaryAssembly, bool bAcceptAbstract = true) where T : class
    {
        if (primaryAssembly == null)
            throw new ArgumentNullException("The primary assembly is unassigned.");

        List<System.Type> retList =
            primaryAssembly.GetTypes().Where(type =>
                typeof(T).IsAssignableFrom(type))
            .ToList();

        List<System.Type> retListFiltered = new List<System.Type>();


        foreach (System.Type typeClass in retList)
        {
            if (bAcceptAbstract || !typeClass.IsAbstract)
            {
                Debug.WriteLine(String.Format("INCLUDED {0} class, abstract value: {1}.", typeClass.Name, typeClass.IsAbstract.ToString()));

                retListFiltered.Add(typeClass);
            }
            else
                Debug.WriteLine(String.Format("IGNORED abstract class: {0}.", typeClass.Name));
        }
        return retListFiltered;
    }