Handle empty tree.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/ht.erl b/src/ht.erl
index bcd9421..3af71f8 100644
--- a/src/ht.erl
+++ b/src/ht.erl
@@ -103,6 +103,8 @@ add(#history_tree{version = V, store = Store}, Entry) ->
 
 tree_hash(Tree) ->
     tree_hash(Tree, Tree#history_tree.version).
+tree_hash(_, -1) ->
+    hash("");
 tree_hash(Tree, Version) ->
     get_hash(Tree, Version, {0, depth(Tree) - 1}).