Loading plugins dynamically using reflection

Here is my code snippet

string pluginDirectory = Path.Combine(
    AppDomain.CurrentDomain.BaseDirectory, "Bin");

var pluginAssemblies =
    from file in new DirectoryInfo(pluginDirectory).GetFiles()
    where file.Extension.ToLower() == ".dll"
    select Assembly.LoadFile(file.FullName);

var pluginTypes =
    from assembly in pluginAssemblies
    from type in assembly.GetExportedTypes()
    where typeof(IPaymentProvider).IsAssignableFrom(type)
    where !type.IsAbstract
    where !type.IsGenericTypeDefinition
    select type;

I get null in the pluginTypes all the time. What am I doing wrong?