Activator.CreateInstance(Type T) returns null

I want to get all classes that implements a specific interface, and show an options list so a user can choose how to implement the interface.

It is a simple interface that describes the character behavior.

I have tried to replace the Interface with an abstract class, but for no avail...

In the debug I can see, that the returned values is null.

All I have found online says that the type might not exist, but it is impossible in my case, because I get my type using reflection.

ICombatBehaviour combat;
private string[] FindAllCombatOptions(out List<Type> existingTypes)
{
    List<string> options = new List<string>();
    existingTypes = new List<Type>();
    var combatBehaviours = Assembly.GetAssembly(typeof(ICombatBehaviour)).GetTypes()
        .Where(myType => myType.IsClass && !myType.IsAbstract && typeof(ICombatBehaviour).IsAssignableFrom(myType));
    foreach (var behaviour in combatBehaviours)
    {
        existingTypes.Add(behaviour);
        options.Add(behaviour.Name);
    }
    return options.ToArray();
}
private void UpdateType(List<Type> types)
{
    combat = Activator.CreateInstance(types[index]) as ICombatBehaviour;
}

The UpdateType() method, gets all the types that FindAllCombatOptions() method has found, and takes the type in the selected index. The type is correct, but the combat variable remains null...