[chuck-users] late binding

Hi Chuckists, I'd like to dynamically switch instrument as in my small code (chuck instr.ck my_chuck.ck) // instr.ck Class MyInstrument { StkInstrument@ m_whatever; Public void Play(dur duration) { m_whatever.noteOn(1); duration => now; m_whatever.noteOff(1); } } // my_chuck.ck MyInstrument dummy; Mandolin man => dummy.m_whatever; Dummy.m_whatever => dac; 1::second => dur d; dummy.Play(d); Then I got this error: [chuck](VM): NullPointerException: (UGen link) in shred[id=5:my_chuck.ck], PC=[46] Where am I wrong with this late binding? Thanks, Beinan