diff --git a/include/sys/types.h b/include/sys/types.h
index 71636dd1a05..51b190ae322 100644
--- a/include/sys/types.h
+++ b/include/sys/types.h
@@ -317,6 +317,10 @@ typedef CODE int (*main_t)(int argc, FAR char *argv[]);
 
 /* POSIX-like OS return values: */
 
+#ifdef OK
+#  undef OK
+#endif
+
 enum
 {
   ERROR = -1,