diff --git a/src/treq/auth.py b/src/treq/auth.py
index d693ff33..e253d39c 100644
--- a/src/treq/auth.py
+++ b/src/treq/auth.py
@@ -28,7 +28,7 @@ def request(self, method, uri, headers=None, bodyProducer=None):
 
 def add_basic_auth(agent, username, password):
     creds = base64.b64encode(
-        '{0}:{1}'.format(username, password).encode('ascii'))
+        u'{}:{}'.format(username, password).encode('utf8'))
     return _RequestHeaderSettingAgent(
         agent,
         Headers({b'Authorization': [b'Basic ' + creds]}))