C# 6.0 primary constructor accessibility

Program.cs

class Program
{
    static void Main()
    {
        new PublicConstructor(""); // OK
        new PrivateConstructor(""); // error CS0122: 'Program.PrivateConstructor.PrivateConstructor(string)' is inaccessible due to its protection level
        new PrimaryConstructor(""); // OK
    }

    private class PrimaryConstructor(string firstName)
    {
    }

    private class PublicConstructor
    {
        public PublicConstructor(string firstName)
        {
        }
    }

    private class PrivateConstructor
    {
        private PrivateConstructor(string firstName)
        {
        }
    }
}