diff --git a/io.c b/io.c
index b1e6c77a3a..7670a6995f 100644
--- a/io.c
+++ b/io.c
@@ -2425,7 +2425,7 @@ rb_io_modenum_mode(flags)
     int flags;
 {
 #ifdef O_BINARY
-# define MODE_BINARY(a,b) ((mode & O_BINARY) ? (a) : (b))
+# define MODE_BINARY(a,b) ((flags & O_BINARY) ? (a) : (b))
 #else
 # define MODE_BINARY(a,b) (a)
 #endif