diff --git a/lib/grit/index.rb b/lib/grit/index.rb
index 2f602f76..c358979a 100644
--- a/lib/grit/index.rb
+++ b/lib/grit/index.rb
@@ -186,6 +186,7 @@ def write_tree(tree = nil, now_tree = nil)
               mode = mode.to_i.to_s  # leading 0s not allowed
               k = k.split('/').last  # slashes not allowed
               str = "%s %s\0%s" % [mode, k, sha]
+              k += '/' if mode == "40000"
               tree_contents[k] = str
             end
           when String