diff --git a/include/binary-io.h b/include/binary-io.h
index 2f4be43a7c1..beeb0f76c25 100644
--- a/include/binary-io.h
+++ b/include/binary-io.h
@@ -35,7 +35,7 @@
 # undef O_BINARY
 # undef O_TEXT
 #endif
-#if O_BINARY
+#if defined(O_BINARY) && O_BINARY
 # if defined __EMX__ || defined __DJGPP__ || defined __CYGWIN__
 #  include <io.h> /* declares setmode() */
 # else