diff --git a/lib/rb/ext/extconf.rb b/lib/rb/ext/extconf.rb
index ed20f56..96f6b43 100644
--- a/lib/rb/ext/extconf.rb
+++ b/lib/rb/ext/extconf.rb
@@ -24,7 +24,7 @@
 
   $ARCH_FLAGS = Config::CONFIG['CFLAGS'].scan( /(-arch )(\S+)/ ).map{|x,y| x + y + ' ' }.join('')
 
-  $CFLAGS = "-g -O2 -Wall -Werror " + $ARCH_FLAGS
+  $CFLAGS = "-fsigned-char -g -O2 -Wall -Werror " + $ARCH_FLAGS
 
   have_func("strlcpy", "string.h")