Abstract Class as ref compiler error

Question

So I've created a framework for settings in my application, all been good until I came across this little roadbump.

In the abstract base class I have this:

public abstract class SettingsBase
{
    public event SettingsChangedEventHandler SettingsChanged;

    public void NotifyChange() { NotifyChange(this); }

    private void NotifyChange(object sender, EventArgs args = null) 
    { 
        SettingsChanged?.Invoke(this); 
    }

    protected void OnSetChildSettings(ref SettingsBase settings, SettingsBase value)
    {
        if (Equals(settings, value)) return;
        if (settings != null) settings.SettingsChanged -= NotifyChange;
        settings = value;
        if (settings != null) settings.SettingsChanged += NotifyChange;
    }
}

Which is a cut down version showing just the code that's causing me grief.

And this in one implementation:

public class ChildSettings : SettingsBase { }

And here is the where the compiler error shows up:

public class ParentSettings : SettingsBase
{
    private ChildSettings _childSettings;

    public ChildSettings ChildSettings
    {
        get { return _childSettings; }
        set { OnSetChildSettings(ref _childSettings, value); }
    }
}

It's driving me mad, saying: "The 'ref' parameter type doesn't match parameter type". It's definitely a SettingsBase, do I have do a cast? Do I have to do template with constraints? This is very poor of C# to not work if so, though maybe I am just spoiled after using it for a few years now.

Edit - This works:

protected void OnSetChildSettings<T>(ref T settings, T value) where T : SettingsBase
{
    if (Equals(settings, value)) return;
    if (settings != null) settings.SettingsChanged -= NotifyChange;
    settings = value;
    if (settings != null) settings.SettingsChanged += NotifyChange;
}

Although I have a working solution I still want to understand what was wrong and why, so answers are still greatly appreciated.

Edit2 - Fixed naming, I just changed it to simply things for this question and missed some. Thanks :)