diff --git a/index.js b/index.js
index 25943502..83c69c2c 100644
--- a/index.js
+++ b/index.js
@@ -497,7 +497,7 @@ function parse (args, opts) {
       o[key] = increment(o[key])
     } else if (o[key] === undefined && checkAllAliases(key, flags.arrays)) {
       o[key] = Array.isArray(value) ? value : [value]
-    } else if (o[key] === undefined || typeof o[key] === 'boolean') {
+    } else if (o[key] === undefined || checkAllAliases(key, flags.bools)) {
       o[key] = value
     } else if (Array.isArray(o[key])) {
       o[key].push(value)
diff --git a/test/yargs-parser.js b/test/yargs-parser.js
index 00e5b53d..0858b19d 100644
--- a/test/yargs-parser.js
+++ b/test/yargs-parser.js
@@ -879,7 +879,7 @@ describe('yargs-parser', function () {
     })
 
     describe('for boolean options', function () {
-      [true, false, undefined].forEach(function (def) {
+      [true, false, undefined, null].forEach(function (def) {
         describe('with explicit ' + def + ' default', function () {
           var opts = {
             default: {