error display variable value/type

Hello everyone,

I am using the following commands in Windbg to debug the following simple application, why there is error like, "Note: this object has an invalid CLASS field
Invalid object"?

[Code]
0:004> ~0e!clrstack -a
OS Thread Id: 0x1194 (0)
Child-SP         RetAddr          Call Site
RSP/REG          Object           Name
RSP/REG          Object           Name
RSP/REG          Object           Name
RSP/REG          Object           Name
RSP/REG          Object           Name
000000000012efe0 0000064280150182 TestDebugManaged1.Program.foo()
    PARAMETERS:
        this = 0x0000000002651ac8
    LOCALS:
        0x000000000012f000 = 0x0000000000000064
        0x000000000012f004 = 0x0000000000000001

RSP/REG          Object           Name
000000000012f020 000006427f67d4a2 TestDebugManaged1.Program.Main(System.String[])
    PARAMETERS:
        args = 0x0000000002651aa8
    LOCALS:
        0x000000000012f040 = 0x0000000002651ac8

0:004> !do 0x000000000012f000
<Note: this object has an invalid CLASS field>
Invalid object

0:004> !do 0x000000000012f004
<Note: this object has an invalid CLASS field>
Invalid object

namespace TestDebugManaged1
{
    class Program
    {
        void foo()
        {
            int a = 100;
            while (true)
            {
                Thread.Sleep(10000);
                a++;
            }
        }
       
        static void Main(string[] args)
        {
            Program instance = new Program();
            instance.foo();
            return;
        }
    }
}
[/Code]

thanks in advance,
George