How to Add Accessibility Checker to TinyMCE

TinyMCE offers an accessibility checker plugin, but Episerver doesn't seem to recognize it. Instead, it shows an error saying that the "a11ychecker" plugin failed to load.

context.Services.Configure<TinyMceConfiguration>(config =>
{
    config.Default()
        .AddPlugin("a11ychecker")
        .Toolbar("a11ycheck");
});

Is this a bug in Episerver, or is there another way to add the a11ycheck button to the toolbar?