diff --git a/common.h b/common.h
index c8399e1..f94b743 100644
--- a/common.h
+++ b/common.h
@@ -5,6 +5,7 @@
  * enough for the macros to adapt (http://support.microsoft.com/kb/111855)
  */
 #ifdef __CYGWIN__
+#undef FD_SETSIZE
 #define FD_SETSIZE 4096
 #endif