Incompatible field Types

Hi All,
  Below r the e.g classes that I m having problem with.

public  class A {
   public String f;

   public A(B b)
   {
	f = b.g;
   }
   public static void main(String arg[])
   {
	A a = new A(new B);
   }

}

public class B {
   public String g;
   public B()
   {
	g = "TEST";
   }
}

The statement f = b.g compiles into

getfield g of B
putfield f of A

I am changing these 2 instructions into

INVOKE* method //which returns object of Object type
checkcast java/lang/String
putfield f of A

But this gives VerifyError saying : Incompatible type for
getting or setting field

Please let me know whether I m missing something over here.

Thanks and Regards
Shrinivas Joshi