Changing Staff Type from "Perc. N lines" to "Custom X" doesn't stick

Repro:
1) Create a new score with an instrument using "Perc 1 line" (or 3 or 5 lines)
2) Using the Instruments dialog, change the Staff Type to "Custom Percussion" (or other Custom)
3) Press Ok to close the dialog
4) Observe no change in score
5) Re-open Instruments dialog
6) Observe no change to staff type (ie, still original "Perc 1 line" type)