diff --git a/lib/fs.js b/lib/fs.js
index 1fd40b9f4fcada..9aa4eaa6a562ec 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -536,8 +536,8 @@ fs.readFileSync = function(path, options) {
 
 // Used by binding.open and friends
 function stringToFlags(flag) {
-  // Only mess with strings
-  if (typeof flag !== 'string') {
+  // Return early if it's a number
+  if (typeof flag === 'number') {
     return flag;
   }
 
diff --git a/test/parallel/test-fs-open-flags.js b/test/parallel/test-fs-open-flags.js
index 16b3d35cae5a49..cf451b4f8d5d95 100644
--- a/test/parallel/test-fs-open-flags.js
+++ b/test/parallel/test-fs-open-flags.js
@@ -34,3 +34,18 @@ assert.equal(fs._stringToFlags('xa+'), O_APPEND | O_CREAT | O_RDWR | O_EXCL);
   .forEach(function(flags) {
     assert.throws(function() { fs._stringToFlags(flags); });
   });
+
+assert.throws(
+  () => fs._stringToFlags({}),
+  /Unknown file open flag: \[object Object\]/
+);
+
+assert.throws(
+  () => fs._stringToFlags(true),
+  /Unknown file open flag: true/
+);
+
+assert.throws(
+  () => fs._stringToFlags(null),
+  /Unknown file open flag: null/
+);