[REG 5.8.0 -> 5.9.0 Beta 3] qmake fails: msvc-version.conf loaded but QMAKE_MSC_VER isn't set

1. I have Qt installed in D:\Qt-online
   So qmake is in D:\Qt-online\5.9\msvc2015_64\bin\qmake.exe
2. I have checked out Creator's super-repository in C:\Users\rob\dev\src\creator-4.3-enterprise
3. I started a MSVC2015 64 bit command line.
4. I ran qtenv2.bat in this:

   C:\Users\rob\dev\59>D:\Qt-online\5.9\msvc2015_64\bin\qtenv2.bat
   Setting up environment for Qt usage...
   Remember to call vcvarsall.bat to complete environment setup!
   

5. I created and changed into the directory that Creator uses as build directory for a test project: C:\Users\rob\dev\src\creator-4.3-enterprise\qtcreator\tests\manual\debugger\build-simple-Qt_5_9_0_msvc2015_64-Debug
6. I'm trying to run qmake on the said project.
   qmake fails:

   C:\Users\rob\dev\src\creator-4.3-enterprise\qtcreator\tests\manual\debugger\build-simple-Qt_5_9_0_msvc2015_64-Debug>qmake C:\Users\rob\dev\src\creator-4.3-enterprise\qtcreator\tests\manual\debugger\simple\simple.pro
   Project ERROR: msvc-version.conf loaded but QMAKE_MSC_VER isn't set
   

Doing the same with Qt 5.8 succeeds. Doing the same in a different directory ( C:\Users\rob\dev\59 ) succeeds with Qt 5.9, too.

qmake should be able to build in any directory, at least in those that Qt 5.8 could handle and are being used by Creator by default.