[DotGNU]unresolved type reference

[Haran Shivanan]

class whatever
{
public static void Main()
{
String s="sdfsdf";
String t="sdf";
s+=t;
}
}

The following, when compiled, says:
[mscorlib]String.Delegate : unresolved type reference

Whats going on?
(Putting s=s+t works though)