IL Emit Disconnected Context Error

I'm trying to make a setter in IL emit, but this setter must only accept objects which are not null, if not, it must throw an exception. This is my class Notnull which recieves a il Generator and a FieldBuilder in which the setter (if not null) must store its value.

 public class NonNull 
{
    public void CreateIL(ILGenerator il, FieldBuilder field)
    {
        il.Emit(OpCodes.Ldarg_0);        // Push "this" on the stack

        il.Emit(OpCodes.Newobj, typeof(Utils).GetConstructor(new Type[] { }));
        il.Emit(OpCodes.Stloc_1);

        il.Emit(OpCodes.Ldarg_1);
        il.Emit(OpCodes.Stloc_2);

        il.Emit(OpCodes.Ldloc_2);
        il.Emit(OpCodes.Ldloc_1);        // Push "value" on the stack
        il.Emit(OpCodes.Callvirt,typeof(Utils).GetMethod("checkIfNull"));
        il.Emit(OpCodes.Pop);

        il.Emit(OpCodes.Ldloc_2);
        il.Emit(OpCodes.Stfld, field);   // Set the field "_Name" to "value"
        il.Emit(OpCodes.Ret);
    }
}

Class utils which I call in the IL and throws my exception.

class Utils
    {
        public void checkIfNull(object obj)
        {
            if (obj == null) throw new MyException();
        }
    }

When I try to create my new type (which has this IL) it presents a Disconnected context error.