After comparing citrus.gemspec, this patch made dh-make-ruby to work. I'm not sure if original gemspec is valid ruby syntax.
diff -ru mustermann-1.0.0/mustermann.gemspec
mustermann-1.0.0+debian/mustermann.gemspec
--- mustermann-1.0.0/mustermann.gemspec 2017-06-28 12:54:46.000000000 +0530
+++ mustermann-1.0.0+debian/mustermann.gemspec 2017-06-28
13:10:06.138765241 +0530
@@ -1,4 +1,4 @@
-$:.unshift File.expand_path("../lib", __FILE__)
+$:.unshift(File.expand_path("../lib", __FILE__))
require "mustermann/version"
Gem::Specification.new do |s|
signature.asc
Description: OpenPGP digital signature
