diff --git a/Yesod/Auth/OAuth2.hs b/Yesod/Auth/OAuth2.hs
--- a/Yesod/Auth/OAuth2.hs
+++ b/Yesod/Auth/OAuth2.hs
@@ -38,6 +38,7 @@
 import Yesod.Form
 
 import qualified Data.ByteString.Lazy as BL
+import qualified Data.ByteString.Char8 as C8
 
 -- | Provider name and Aeson parse error
 data YesodOAuth2Exception = InvalidProfileResponse Text BL.ByteString
@@ -87,7 +88,7 @@
         return oauth
             { oauthCallback = Just $ encodeUtf8 $ render $ tm url
             , oauthOAuthorizeEndpoint = oauthOAuthorizeEndpoint oauth
-                <> "&state=" <> encodeUtf8 csrfToken
+                `appendQuery` "state=" <> encodeUtf8 csrfToken
             }
 
     dispatch "GET" ["forward"] = do
@@ -141,3 +142,9 @@
 
 bsToText :: ByteString -> Text
 bsToText = decodeUtf8With lenientDecode
+
+appendQuery :: ByteString -> ByteString -> ByteString
+appendQuery url query =
+    if '?' `C8.elem` url
+        then url <> "&" <> query
+        else url <> "?" <> query
diff --git a/yesod-auth-oauth2.cabal b/yesod-auth-oauth2.cabal
--- a/yesod-auth-oauth2.cabal
+++ b/yesod-auth-oauth2.cabal
@@ -1,5 +1,5 @@
 name:            yesod-auth-oauth2
-version:         0.1.4
+version:         0.1.5
 license:         BSD3
 license-file:    LICENSE
 author:          Tom Streller