GetTypes() with optional dependencies

I'm working on a project which can have optional external DLL dependencies (I can not access the code of those dependencies). I moved all optional dependency code to its own class, the class is only instantiated when the DLLs are present, so I do not get any issues on runtime.

However, I want to use reflection with Assembly.GetTypes() for some other stuff, but since this tries to load said classes, I get the following exception:

System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded.

If the optional dependencies are loaded, I do not get such an error, since the classes can get loaded.

The related code:

        foreach (var pl in R.Plugins.GetPlugins())
        {
            //pl has the optional dependencies  
            var assembly = pl.GetType().Assembly;
            var types = pl.GetTypes(); //here is the exception
        }

How can I use GetTypes() without loading that specific class that depends on optional dependencies?