Dropping a clef onto a former drum staff hangs MuseScore

Steps to reproduce:
1. Open the small percussion example used by nightly builds
2. select a measure, right click it, access the "staff properties" item; the edit staff/part properties dialog opens
3. click the change instrument button, chose a non-percussive instrument, I choose Tuba; uncheck the "use drumset" checkbox as well
4. back in the score, expand the clefs palette, drag a bass clef and drop it over the percussion clef

Result: MuseScore hangs
Expected result: the program should continue to be in an operable state

Reproduced using: r2821, Windows XP Proffessional, SP 3, AMD 64 dual core, 1.80 Ghz, 1.8 Gb of RAM