diff --git a/include/sys/ttydefaults.h b/include/sys/ttydefaults.h
index d251b715..e5d4da2b 100644
--- a/include/sys/ttydefaults.h
+++ b/include/sys/ttydefaults.h
@@ -6,7 +6,7 @@
 #define TTYDEF_LFLAG (ECHO | ICANON | ISIG | IEXTEN | ECHOE|ECHOKE|ECHOCTL)
 #define TTYDEF_CFLAG (CREAD | CS7 | PARENB | HUPCL)
 #define TTYDEF_SPEED (B9600)
-#define CTRL(x) (x&037)
+#define CTRL(x) ((x)&037)
 #define CEOF CTRL('d')
 
 #ifdef _POSIX_VDISABLE