diff --git a/Cakefile b/Cakefile
index d23d8ff..f0d15a6 100644
--- a/Cakefile
+++ b/Cakefile
@@ -20,7 +20,7 @@ task 'test', ->
       ssl_cert ->
         test_cases = glob.sync 'test/js/**/*_test.js'
         test_cases.sort()  # Consistent test case order.
-        run 'node_modules/.bin/mocha --colors --slow 200 --timeout 1000 ' +
+        run 'node_modules/.bin/mocha --colors --slow 200 --timeout 1000 --exit ' +
             "--require test/js/helpers/setup.js #{test_cases.join(' ')}"
 
 task 'webtest', ->
diff --git a/src/001-xml_http_request.coffee b/src/001-xml_http_request.coffee
index 3e448b4..a37a073 100644
--- a/src/001-xml_http_request.coffee
+++ b/src/001-xml_http_request.coffee
@@ -379,7 +379,6 @@ class XMLHttpRequest extends XMLHttpRequestEventTarget
       trailer: true
       'transfer-encoding': true
       upgrade: true
-      'user-agent': true
       via: true
 
   # HTTP response headers that should not be exposed according to the XHR spec.
@@ -391,7 +390,7 @@ class XMLHttpRequest extends XMLHttpRequestEventTarget
     'set-cookie': true
     'set-cookie2': true
 
-  # The value of the User-Agent header.
+  # The default value of the User-Agent header.
   _userAgent: "Mozilla/5.0 (#{os.type()} #{os.arch()}) " +
               "node.js/#{process.versions.node} v8/#{process.versions.v8}"
 
@@ -483,7 +482,7 @@ class XMLHttpRequest extends XMLHttpRequestEventTarget
     @_headers['Host'] = @_url.host
     if @_anonymous
       @_headers['Referer'] = 'about:blank'
-    @_headers['User-Agent'] = @_userAgent
+    @_headers['User-Agent'] ||= @_userAgent
     @upload._finalizeHeaders @_headers, @_loweredHeaders
     undefined
 
diff --git a/test/src/headers_test.coffee b/test/src/headers_test.coffee
index 4ba33a8..09e038b 100644
--- a/test/src/headers_test.coffee
+++ b/test/src/headers_test.coffee
@@ -10,6 +10,7 @@ describe 'XMLHttpRequest', ->
     describe 'with allowed headers', ->
       beforeEach ->
         @xhr.setRequestHeader 'Authorization', 'lol'
+        @xhr.setRequestHeader 'User-Agent', 'toaster'
         @xhr.setRequestHeader 'X-Answer', '42'
         @xhr.setRequestHeader 'X-Header-Name', 'value'
 
@@ -19,6 +20,8 @@ describe 'XMLHttpRequest', ->
           headers = JSON.parse @xhr.responseText
           expect(headers).to.have.property 'authorization'
           expect(headers['authorization']).to.equal 'lol'
+          expect(headers).to.have.property 'user-agent'
+          expect(headers['user-agent']).to.equal 'toaster'
           expect(headers).to.have.property 'x-answer'
           expect(headers['x-answer']).to.equal '42'
           expect(headers).to.have.property 'x-header-name'