Implicit cast of Enum in method arguments

Question

I do not understand the logic of the implicit casting of the int to Enum.

  class Program
{
    static void Main(string[] args)
    {
       TestMethod((int)0); //ok
        TestMethod((int)1); //compile error it is not assignable
    }

    private static void TestMethod(SuperEnum e)
    {
       //...
    }
}

public enum SuperEnum : int
{
    None = 0,
    One,
    Two
}

Why zero is acceptable but any integers are not?

The MSIL listing of the first line looks the same for

TestMethod(0) and TestMethod((SuperEnum)1)

 // [12 9 - 12 10]
IL_0000: nop          

// [13 12 - 13 31]
IL_0001: ldc.i4.0  //load 0 or 1   
IL_0002: call         void ConsoleApp2.Program::TestMethod(valuetype ConsoleApp2.SuperEnum)
IL_0007: nop          

// [15 9 - 15 10]
IL_0008: ret          

Thanks!