adding a kit to a project is broken

How to reproduce:

• open a project
• add a kit that wasn't in the project before
• try to build the project

Symptoms:
It won't build, with the following error message:

13:16:40: Running steps for project demoLauncher...
13:16:40: Could not start process "D:\dev\qt\stable-vs2012\qtbase\bin\qmake.exe" D:\bugreports\QTBUG\31333\demoLauncher\demoLauncher.pro -r -spec win32-msvc2012 "CONFIG+=declarative_debug" "CONFIG+=qml_debug"
Error while building/deploying project demoLauncher (kit: Qt stable-vs2012)
When executing step 'qmake'
13:16:40: Elapsed time: 00:00.

This is because the build directory is never created.
After creating the directory manually, it builds successfully.