Is the name Secrets an invalid name for a class in C#

I am probably making a dumb error but I cannot see it.

I added a custom class to my project following the simple Newtownsoft example here.

namespace MyApp
{
     public class Secrets
     {
          public string Test {get;set;}
     } 
}

But when I go to reference the class in my Program.cs:

      namespace MyApp
      {
          static class Program
           {
              [STAThread]
              static void Main()
               {
                 <snip>

                 string json = FetchJson();
                 var MySecrets = JsonConvert.DeserializeObject<Secrets>(json);   // this line
               }
           }
      }
 

Visual Studio underlines <Secrets> and says the type or namespace 'Secrets' cannot be found. But the project compiles without error.

If I change the name of the class to Sekrets then Visual Studio finds it and is happy.

P.S. And if I then rename the file from Sekrets.cs back to Secrets.cs and let VS make the global change, then VS remains happy, no error, no red underlining.