diff --git a/ext/pty/extconf.rb b/ext/pty/extconf.rb
index 2696ae0d3b..ba2b44c70b 100644
--- a/ext/pty/extconf.rb
+++ b/ext/pty/extconf.rb
@@ -3,10 +3,9 @@ require 'mkmf'
 if /mswin32|mingw/ !~ RUBY_PLATFORM
   have_header("sys/stropts.h")
   have_func("setresuid")
-#  $CFLAGS << "-DHAVE_DEV_PTMX" if /cygwin/ === RUBY_PLATFORM
+  $CFLAGS << "-DHAVE_DEV_PTMX" if /cygwin/ === RUBY_PLATFORM
   if have_func("openpty") or
       have_func("_getpty") or
-      have_func("ptsname") or
       have_func("ioctl")
     create_makefile('pty')
   end
diff --git a/ext/pty/pty.c b/ext/pty/pty.c
index d71970eb66..35e4080e62 100644
--- a/ext/pty/pty.c
+++ b/ext/pty/pty.c
@@ -359,7 +359,7 @@ getDevice(master,slave)
     int	 i,j;
     char MasterName[DEVICELEN];
 
-#ifdef HAVE_PTSNAME
+#ifdef HAVE_DEV_PTMX
     char *pn;
     void (*s)();