diff --git a/index.js b/index.js
index 349b516..ccfc7f1 100644
--- a/index.js
+++ b/index.js
@@ -68,7 +68,7 @@ function collectMessages(tree) {
                     tree[key].unique = true;
                 } else {
                     // Nested schema
-                    var subtree = collectMessages(tree[key].tree);
+                    var subtree = collectMessages(tree[key].tree || tree[key]);
 
                     for (var subkey in subtree) {
                         if (subtree.hasOwnProperty(subkey)) {