A kit's custom environment messes up the build environment

After inserting a custom environment for my preferred kit, e.g.

INCLUDE=D:\Development9\UnixLibsV9-x64\include;${INCLUDE}
LIB=D:\Development9\UnixLibsV9-x64\lib;${LIB}
PATH=D:\Development9\UnixLibsV9-x64\bin;${PATH}

, the Qt Creator is unable to build my C++ project and complains about missing cl.exe or nmake.exe (depending on the state the project is in).

Since I am using the official installers for Windows, I know that inserting a custom environment worked fine with the Creator of the 5.6.1 installer. The Creator of the 5.9.5 installer messes things up.

I need to use some custom build Unix libraries in several projects I am developing, so it is a lot easier for me to put them in a custom environment than to add them into each .PRO file.

Please fix this for the next official installer!