diff --git a/lib/fs.js b/lib/fs.js
index a5eb5351acfe..5345afa36b50 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -50,10 +50,6 @@ var O_RDWR = constants.O_RDWR || 0;
 var O_SYNC = constants.O_SYNC || 0;
 var O_TRUNC = constants.O_TRUNC || 0;
 var O_WRONLY = constants.O_WRONLY || 0;
-var F_OK = constants.F_OK || 0;
-var R_OK = constants.R_OK || 0;
-var W_OK = constants.W_OK || 0;
-var X_OK = constants.X_OK || 0;
 
 var isWindows = process.platform === 'win32';
 
@@ -186,10 +182,12 @@ fs.Stats.prototype.isSocket = function() {
   return this._checkModeProperty(constants.S_IFSOCK);
 };
 
-fs.F_OK = F_OK;
-fs.R_OK = R_OK;
-fs.W_OK = W_OK;
-fs.X_OK = X_OK;
+// don't allow fs.mode to accidentally be overwritten.
+['F_OK', 'R_OK', 'W_OK', 'X_OK'].forEach(function(key) {
+  Object.defineProperty(fs, key, {
+    enumerable: true, value: constants[key] || 0, writable: false
+  });
+});
 
 fs.access = function(path, mode, callback) {
   if (!nullCheck(path, callback))
@@ -197,7 +195,7 @@ fs.access = function(path, mode, callback) {
 
   if (typeof mode === 'function') {
     callback = mode;
-    mode = F_OK;
+    mode = fs.F_OK;
   } else if (typeof callback !== 'function') {
     throw new TypeError('callback must be a function');
   }
@@ -212,7 +210,7 @@ fs.accessSync = function(path, mode) {
   nullCheck(path);
 
   if (mode === undefined)
-    mode = F_OK;
+    mode = fs.F_OK;
   else
     mode = mode | 0;