Why SOS Debugging Extension can NOT debug Nullable type?

void Main(string[] args)
{
Nullable<int> nullINT = 100;
} 1).load sos.dll 2)!clrstack -l      Cannot trace out the variable info.


But for this following code,

 static void Main(string[] args)
{
SimpleClass sc1 = new SimpleClass();
}

1).load sos.dll 2)!clrstack -l      Can trace out the variable info right now. Looks like this,

LOCALS:

0x0012f420 = 0x0130652c

0x0012f41c = 0x00000000

0x0012f40c = 0x00000000