Transpose by diminished second doesn't work (0.9.6 branch regression)

Steps to reproduce bug
1. Create a new flute score and add a D-flat
2. Edit > Select All
3. Notes > Transpose...
4. Check "Transpose by Interval"
5. Select "Down"
6. Select "Diminished Second"
7. Click "OK" to dismiss dialog

Expected behavior: The D-flat note should transpose to C sharp.

Actual behavior: The D-flat note does not transpose.

Discussion: Marked as critical since it is a significant regression on the stable branch.

MuseScore version: Bug present in 0.9.6.3 release candidate. Bug not present in 0.9.6

(Operating System: Windows 7)