Irregular measure check mark doesn't stay marked (0.9.6 regression)

Steps to reproduce bug
1. Open a score with a pickup (such as Happa Haunt .mscz )
2. Right click on an empty part of the pickup measure and choose "Measure Properties..."
3. Add a check mark next to "irregular"
4. Press OK

Expected behavior: Measure numbers should change as described in the "Irregular" section of measure operations .

Actual behavior: Adding a check mark for "irregular" seems to have no effect. If I go back to "Measure properties" the check box has unmarked itself,

MuseScore version: r. 2646

(Operating System: Windows XP)