diff --git a/index.js b/index.js
index 816a15a..9e98b13 100644
--- a/index.js
+++ b/index.js
@@ -66,12 +66,12 @@ function formatOpts(opts) {
   if (!('maxAge' in opts)) opts.maxAge = opts.maxage;
 
   // defaults
-  if (opts.overwrite == null) opts.overwrite = true;
-  if (opts.httpOnly == null) opts.httpOnly = true;
+  if (util.isNil(opts.overwrite)) opts.overwrite = true;
+  if (util.isNil(opts.httpOnly)) opts.httpOnly = true;
   // delete null sameSite config
-  if (opts.sameSite == null) delete opts.sameSite;
-  if (opts.signed == null) opts.signed = true;
-  if (opts.autoCommit == null) opts.autoCommit = true;
+  if (util.isNil(opts.sameSite)) delete opts.sameSite;
+  if (util.isNil(opts.signed)) opts.signed = true;
+  if (util.isNil(opts.autoCommit)) opts.autoCommit = true;
 
   debug('session options %j', opts);
 
diff --git a/lib/util.js b/lib/util.js
index b3a1a28..67c61ca 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -35,5 +35,9 @@ module.exports = {
     return crc(JSON.stringify(sess));
   },
 
+  isNil(val) {
+    return val === null || typeof val === 'undefined';
+  },
+
   CookieDateEpoch: 'Thu, 01 Jan 1970 00:00:00 GMT',
 };