diff --git a/src/Tests/Arbitrary.hs b/src/Tests/Arbitrary.hs
index f5a44ceab..3ad90020d 100644
--- a/src/Tests/Arbitrary.hs
+++ b/src/Tests/Arbitrary.hs
@@ -1,5 +1,5 @@
 {-# OPTIONS_GHC -fno-warn-orphans #-}
-{-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-}
 -- provides Arbitrary instance for Pandoc types
 module Tests.Arbitrary ()
 where