diff --git a/brat/Brat/Load.hs b/brat/Brat/Load.hs
index e427246..f64437f 100644
--- a/brat/Brat/Load.hs
+++ b/brat/Brat/Load.hs
@@ -29,7 +29,7 @@ import Control.Monad (filterM, foldM, forM, forM_, unless)
 import Control.Monad.Except
 import Control.Monad.Trans.Class (lift)
 import Data.Functor ( (<&>), ($>) )
-import Data.List (sort)
+import Data.List (sort, unzip)
 import Data.List.HT (viewR)
 import Data.List.NonEmpty (NonEmpty(..))
 import qualified Data.Graph as G