diff --git a/index.js b/index.js
index 9dc4b53..92ed24f 100644
--- a/index.js
+++ b/index.js
@@ -42,7 +42,7 @@ module.exports = function argsert (typeConfig, ...args) {
     if ('optional' in typesAtIndex) {
       const optional = typesAtIndex.optional;
 
-      if (optional.indexOf('*') < 0 && optional.indexOf(observedType) < 0) {
+      if (arg !== undefined && optional.indexOf('*') < 0 && optional.indexOf(observedType) < 0) {
         throw new TypeError(errorMessage('optional'));
       }
     }
@@ -91,7 +91,9 @@ function positionName (index) {
 }
 
 function expectedTypes (types, kind) {
-  return types[kind].join(' or ');
+  return types[kind]
+    .concat(kind === 'optional' && types[kind].indexOf('undefined') < 0 ? 'undefined' : [])
+    .join(' or ');
 }
 
 function isOptional (arg) {
diff --git a/test.js b/test.js
index b22ee8b..ca1c3c9 100644
--- a/test.js
+++ b/test.js
@@ -10,11 +10,11 @@ test('does not throw exception if optional argument is not provided', async t =>
 test('throws exception if wrong type is provided for optional argument', t => {
   t.throws(
     () => argsert('[object|number]', 'hello'),
-    /Invalid first argument. Expected object or number but received string./
+    /Invalid first argument. Expected object or number or undefined but received string./
   );
   t.throws(
     argsertPromise('[object|number]', 'hello'),
-    /Invalid first argument. Expected object or number but received string./
+    /Invalid first argument. Expected object or number or undefined but received string./
   );
 });
 
@@ -22,10 +22,10 @@ test('does not throw exception if optional argument is valid', t => {
   t.true(argsert('[object]', {foo: 'bar'}));
 });
 
-test('throws exception if required argument is not provided', t => {
+test('throws exception if required arguments are not provided', t => {
   t.throws(
-    () => argsert('<object>'),
-    /Not enough arguments provided. Expected 1 but received 0./
+    () => argsert('<object> <*>'),
+    /Not enough arguments provided. Expected 2 but received 0./
   );
 });
 
@@ -85,6 +85,10 @@ test('allows wildcard to be used in optional configuration', async t => {
   t.true(await argsertPromise('[string] [*]', 'foo', {}));
 });
 
+test('allows undefined for optional arguments', t => {
+  t.true(argsert('[string] <*>', undefined, {}));
+});
+
 test('throws when the optional config has broken syntax', t => {
   t.throws(
     () => argsert('<*> [foo||bar]', 'baz', 'boo'),
@@ -119,6 +123,18 @@ test('allows rejected promise in the optional config', t => {
   t.true(argsert('[promise]', Promise.reject(new Error('no timeout')).catch(() => {})));
 });
 
+test('allows buffer in the required config', t => {
+  const buffer = new Buffer('robin');
+
+  t.true(argsert('<buffer>', buffer));
+});
+
+test('allows buffer in the optional config', t => {
+  const buffer = new Buffer('batgirl');
+
+  t.true(argsert('[buffer]', buffer));
+});
+
 test('the arguments object can be passed in, spread', t => {
   function foo () {
     return argsert('[string|number] <object>', ...arguments);
@@ -127,14 +143,21 @@ test('the arguments object can be passed in, spread', t => {
   t.true(foo('far', {}));
 });
 
-test('allows buffer in the required config', t => {
-  const buffer = new Buffer('robin');
+test('Function.prototype.apply with the arguments object', t => {
+  function foo () {
+    return argsert.apply(this, arguments);
+  }
 
-  t.true(argsert('<buffer>', buffer));
+  t.true(foo('[string|number] <object>', 'bar', {}));
 });
 
-test('allows buffer in the optional config', t => {
-  const buffer = new Buffer('batgirl');
+test('Function.prototype.call with the arguments object spread', t => {
+  function foo () {
+    return argsert.call(this, ...arguments);
+  }
 
-  t.true(argsert('[buffer]', buffer));
+  t.throws(
+    () => foo('[number] <object>', 'bar', {}),
+    /Invalid first argument. Expected number or undefined but received string./
+  );
 });