ASP.NET MVC 3 Tools Update Setup Fails with Fatal Error (0x80070643)

Today, I tried to install the ASP.NET MVC 3 Tools Update. The installation always failed with a fatal error(0x80070643). The log file sent me to the ASP.NET Webpages.msi logfile which had the wonderful error message

There is a problem with this Windows Installer package. A program run as part of the setup did not finish as expected. Contact your support personnel or package vendor.

The Eventlog had even some additional but equally useless information:

Application: WebConfigCA.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.ArgumentException
Stack:
at System.Web.Configuration.VirtualDirectoryMapping..ctor(System.Web.VirtualPath, System.String, Boolean, System.String)
at System.Web.Configuration.VirtualDirectoryMapping..ctor(System.String, Boolean)

After some digging, I found a KB article describing the same error during the installation of MVC 3 or WebPages and the first suggested workaround helped:

Remove the trailing backslash from the following Registry keys:

HKEYLOCALMACHINE\SOFTWARE\Microsoft\ASP.NET\4.0.30319.0\Path
HKEYLOCALMACHINE\SOFTWARE\Wow6432Node\Microsoft\ASP.NET\4.0.30319.0\Path

I guess someone at Microsoft missed the Path.Combine method. ;)