diff --git a/swig/ruby/Makefile.am b/swig/ruby/Makefile.am
index 12dd9af..801340c 100644
--- a/swig/ruby/Makefile.am
+++ b/swig/ruby/Makefile.am
@@ -12,10 +12,10 @@ all-local:
 install-data-local:
 endif
 
-ext/Makefile config.save:
+ext/Makefile config.save: $(srcdir)/ext/comedi_ruby_wrap.c
 	$(RUBY) $(srcdir)/setup.rb config $(RUBY_CONFIG_OPTIONS) -- --with-comedilib-dir=../../..
 
-ext/comedi.so ext/comedi_ruby_wrap.o: ext/Makefile config.save $(srcdir)/ext/comedi_ruby_wrap.c
+ext/comedi.so ext/comedi_ruby_wrap.o: ext/Makefile config.save
 	$(RUBY) $(srcdir)/setup.rb setup
 
 if HAVE_RUBY