Thank you, @ccoenen and @tmm1.
You are right. I removed ```then```.

>Why is this guard needed anyway?

Because Rake::ExtensionTask#cross_platform returns String or Array.
Please refer to the link below for details.
https://github.com/luislavena/rake-compiler/blob/master/lib/rake/extensiontask.rb#L32
https://github.com/luislavena/rake-compiler/blob/master/lib/rake/extensiontask.rb#L69