diff --git a/lib/ocran/inno_setup_script_builder.rb b/lib/ocran/inno_setup_script_builder.rb
index 4648f69..6cd8b2c 100644
--- a/lib/ocran/inno_setup_script_builder.rb
+++ b/lib/ocran/inno_setup_script_builder.rb
@@ -47,7 +47,7 @@ def cp(source, target)
         raise "The file does not exist (#{source})"
       end
 
-      return if @files.add?(source, target)
+      return unless @files.add?(source, target)
 
       @_files << {
         source: source,