diff --git a/lib/fs.js b/lib/fs.js
index eeb3806cdc8861..f9f104eb229994 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -465,10 +465,11 @@ fs.readFileSync = function(path, options) {
 
 // Used by binding.open and friends
 function stringToFlags(flag) {
-  // Only mess with strings
-  if (typeof flag !== 'string') {
+  if (typeof flag === 'number')
     return flag;
-  }
+
+  if (typeof flag !== 'string')
+    throw new TypeError('flag must be a string');
 
   switch (flag) {
     case 'r' : return O_RDONLY;
diff --git a/test/parallel/test-fs-open-flags.js b/test/parallel/test-fs-open-flags.js
index c7d53117d587fc..0070e3011d8ba2 100644
--- a/test/parallel/test-fs-open-flags.js
+++ b/test/parallel/test-fs-open-flags.js
@@ -38,3 +38,17 @@ assert.equal(fs._stringToFlags('xa+'), O_APPEND | O_CREAT | O_RDWR | O_EXCL);
  'x +x x+ rx rx+ wxx wax xwx xxx').split(' ').forEach(function(flags) {
   assert.throws(function() { fs._stringToFlags(flags); });
 });
+
+// Use of numeric flags is permitted.
+assert.equal(fs._stringToFlags(O_RDONLY), O_RDONLY);
+
+// Non-numeric/string flags are a type error.
+assert.throws(function() { fs._stringToFlags(undefined); }, TypeError);
+assert.throws(function() { fs._stringToFlags(null); }, TypeError);
+assert.throws(function() { fs._stringToFlags(assert); }, TypeError);
+assert.throws(function() { fs._stringToFlags([]); }, TypeError);
+assert.throws(function() { fs._stringToFlags({}); }, TypeError);
+
+// Numeric flags that are not int will be passed to the binding, which
+// will throw a TypeError
+assert.throws(function() { fs.openSync('_', O_RDONLY + 0.1); }, TypeError);