Creating plugin: Missing *.h.ex

I am trying to create a plugin and have set up the usual folders and files. In particular, I have a (dummy) Module.h and *.cpp derived from IModuleInterface. The build fails with

ERROR: System.IO.FileNotFoundException: Could not find file “[…]/Plugins/MyPlugin/Source/MyPlugin/Private/Module.h.ex”

which certainly isn’t there and probably should somehow be created, but I don’t enough about UBT to see what I’m missing here. This is on OSX 10.10.

Any ideas?