diff --git a/hw-prim.cabal b/hw-prim.cabal
--- a/hw-prim.cabal
+++ b/hw-prim.cabal
@@ -1,5 +1,5 @@
 name:                   hw-prim
-version:                0.0.0.10
+version:                0.0.0.11
 synopsis:               Primitive functions and data types
 description:            Please see README.md
 homepage:               http://github.com/haskell-works/hw-prim#readme
diff --git a/src/HaskellWorks/Data/TreeCursor.hs b/src/HaskellWorks/Data/TreeCursor.hs
--- a/src/HaskellWorks/Data/TreeCursor.hs
+++ b/src/HaskellWorks/Data/TreeCursor.hs
@@ -10,8 +10,8 @@
 import           HaskellWorks.Data.Positioning
 
 class TreeCursor k where
-  firstChild :: k -> k
-  nextSibling :: k -> k
-  parent :: k -> k
+  firstChild :: k -> Maybe k
+  nextSibling :: k -> Maybe k
+  parent :: k -> Maybe k
   depth :: k -> Count
   subtreeSize :: k -> Count