wine cmd.exe /c echo ‘%ProgramFiles%’ returned empty string

I had this problem today:

wine cmd.exe /c echo '%ProgramFiles%' returned empty string

When I was trying to run Winetricks:

sh winetricks msxml6 gdiplus gecko vcrun2005

Until I realised that I was trying to run winetricks as root rather than as my normal unprivileged user. Just in case it helps anyone else.