diff --git a/ares/ares.h b/ares/ares.h
index c3959e0b4f..09efa133fc 100644
--- a/ares/ares.h
+++ b/ares/ares.h
@@ -20,7 +20,10 @@
 
 #include <sys/types.h>
 
-#ifdef HAVE_SYS_SELECT_H
+#ifdef _AIX
+/* HP-UX systems version 9, 10 and 11 lack sys/select.h and so does oldish
+   libc5-based Linux systems. Only include it on system that are known to
+   require it! */
 #include <sys/select.h>
 #endif