diff --git a/README.md b/README.md
index 580eac0..87ef07e 100644
--- a/README.md
+++ b/README.md
@@ -64,7 +64,8 @@ one of the three passed credentials.
 
 Alternatively, you can pass your own `authorizer` function, to check the credentials
 however you want. It will be called with a username and password and is expected to
-return `true` or `false` to indicate that the credentials were approved or not:
+return `true` or `false`, or a `Promise` of `true` or `false` to indicate that the
+credentials were approved or not:
 
 ```js
 app.use(basicAuth( { authorizer: myAuthorizer } ))
@@ -75,8 +76,18 @@ function myAuthorizer(username, password) {
 ```
 
 This will authorize all requests with credentials where the username begins with
-`'A'` and the password begins with `'secret'`. In an actual application you would
-likely look up some data instead ;-)
+`'A'` and the password begins with `'secret'`.
+
+Alternatively, you can use an `async` function or return a promise:
+
+```js
+app.use(basicAuth( { authorizer: myAuthorizer } ))
+
+async function myAuthorizer(username, password) {
+    const user = await database.findUser(username)
+    return user.comparePassword(password)
+}
+```
 
 ### Custom Async Authorization
 
diff --git a/index.js b/index.js
index ee08756..c24d0ae 100644
--- a/index.js
+++ b/index.js
@@ -43,8 +43,20 @@ function buildMiddleware(options) {
 
         if(isAsync)
             return authorizer(authentication.name, authentication.pass, authorizerCallback)
-        else if(!authorizer(authentication.name, authentication.pass))
-            return unauthorized()
+        else {
+            var result = authorizer(authentication.name, authentication.pass)
+            if (result && typeof result.then === 'function') {
+                return result.then(function (isAuthorized) {
+                    authorizerCallback(undefined, isAuthorized)
+                }, function (error) {
+                    authorizerCallback(error)
+                }).catch(function (e) {
+                    console.log('e', e)
+                })
+            } else if (!result) {
+                return unauthorized()
+            }
+        }
 
         return next()
 
@@ -69,7 +81,9 @@ function buildMiddleware(options) {
         }
 
         function authorizerCallback(err, approved) {
-            assert.ifError(err)
+            if (err) {
+                return next(err)
+            }
 
             if(approved)
                 return next()
diff --git a/test.js b/test.js
index 9bf122a..f84a1a7 100644
--- a/test.js
+++ b/test.js
@@ -4,6 +4,7 @@ const express = require('express')
 const supertest = require('supertest');
 
 var app = express()
+app.set('env', 'test')
 
 //Requires basic auth with username 'Admin' and password 'secret1234'
 var staticUserAuth = basicAuth({
@@ -30,6 +31,11 @@ var asyncAuth = basicAuth({
     authorizeAsync: true
 })
 
+//Uses a custom promise-based authorizer function
+var promiseAuth = basicAuth({
+    authorizer: myPromiseAuthorizer
+})
+
 //Uses a custom response body function
 var customBodyAuth = basicAuth({
     users: { 'Foo': 'bar' },
@@ -76,6 +82,10 @@ app.get('/async', asyncAuth, function(req, res) {
     res.status(200).send('You passed')
 })
 
+app.get('/promise', promiseAuth, function(req, res) {
+    res.status(200).send('You passed')
+})
+
 app.get('/custombody', customBodyAuth, function(req, res) {
     res.status(200).send('You passed')
 })
@@ -98,15 +108,34 @@ app.get('/realmfunction', realmFunctionAuth, function(req, res) {
 
 //Custom authorizer checking if the username starts with 'A' and the password with 'secret'
 function myAuthorizer(username, password) {
-    return username.startsWith('A') && password.startsWith('secret')
+    if(username.startsWith('A') && password.startsWith('secret'))
+        return true
+    else if (username.startsWith('error'))
+        throw new Error('authorizer error')
+    else
+        return false
 }
 
 //Same but asynchronous
 function myAsyncAuthorizer(username, password, cb) {
+    setTimeout(function () {
+        if(username.startsWith('A') && password.startsWith('secret'))
+            return cb(null, true)
+        else if (username.startsWith('error'))
+            return cb(new Error('authorizer error'))
+        else
+            return cb(null, false)
+    }, 1)
+}
+
+//Same but returns promise
+function myPromiseAuthorizer(username, password) {
     if(username.startsWith('A') && password.startsWith('secret'))
-        return cb(null, true)
+        return Promise.resolve(true)
+    else if (username.startsWith('error'))
+        return Promise.reject(new Error('authorizer error'))
     else
-        return cb(null, false)
+        return Promise.resolve(false)
 }
 
 function getUnauthorizedResponse(req) {
@@ -165,6 +194,13 @@ describe('express-basic-auth', function() {
                 .expect(401, done)
         })
 
+        it('should return 500 if authoriser rejects', function(done) {
+            supertest(app)
+                .get(endpoint)
+                .auth('error', 'stuff')
+                .expect(500, done)
+        })
+
         it('should accept fitting credentials', function(done) {
             supertest(app)
                 .get(endpoint)
@@ -189,6 +225,44 @@ describe('express-basic-auth', function() {
                 .expect(401, done)
         })
 
+        it('should return 500 if authoriser rejects', function(done) {
+            supertest(app)
+                .get(endpoint)
+                .auth('error', 'stuff')
+                .expect(500, done)
+        })
+
+        it('should accept fitting credentials', function(done) {
+            supertest(app)
+                .get(endpoint)
+                .auth('Aererer', 'secretiveStuff')
+                .expect(200, 'You passed', done)
+        })
+    })
+
+    describe('promise authorizer', function() {
+        const endpoint = '/promise'
+
+        it('should reject on missing header', function(done) {
+            supertest(app)
+                .get(endpoint)
+                .expect(401, done)
+        })
+
+        it('should reject on wrong credentials', function(done) {
+            supertest(app)
+                .get(endpoint)
+                .auth('dude', 'stuff')
+                .expect(401, done)
+        })
+
+        it('should return 500 if authoriser rejects', function(done) {
+            supertest(app)
+                .get(endpoint)
+                .auth('error', 'stuff')
+                .expect(500, done)
+        })
+
         it('should accept fitting credentials', function(done) {
             supertest(app)
                 .get(endpoint)