Problem building VSIX with MSBuild 2017

I have upgraded my Package to Visual Studio 2017. In my build process I build a release using the MSBuild command, something like:

<path>\MSBuild.exe <solution file> /t:Build /p:Configuration=Release

If I use this command with Visual Studio 2017 RC, then it appears to build correctly, but the VSIX file it generates is invalid. On trying to install the package I get the following error message.

If I build the package using DevEnv with the command line

<path>devenv.exe <solution file> /Rebuild Release

then it also builds the package, but in this case the package is valid

I have two questions:

• Am I doing something wrong with, or is it a bug in Visual Studio 2017 RC
• What does the message "The file is not a valid VSIX package" actually mean?
  Is there any way to debug it?

Phil