Project fails to compile after 2015 Update 1

I've just applied Update 1 of the 2015 toolset and now cannot even build projects. I'm getting the following message:

C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V120\Platforms\x64\PlatformToolsets\Intel C++ Compiler XE 15.0\Toolset.targets(150,5): error : Could not expand ICInstallDir variable. Platform toolset may be set to an invalid version number.