diff --git a/src/tree.rs b/src/tree.rs
index a648d90..6165e1d 100644
--- a/src/tree.rs
+++ b/src/tree.rs
@@ -744,13 +744,21 @@ impl<K, V> NodeRef<marker::Owned, K, V> {
         // insert new key and child node.
         // SAFETY: we just grew the allocations.
         unsafe {
-            assert!(new_len == parent.idx + 1);
             slice_insert(parent.node.key_area_mut(..new_len), parent.idx, key);
             slice_insert(parent.node.edge_area_mut(..new_len), parent.idx, self.node);
         }
 
         *parent.node.len_mut() = new_len as u16;
 
+        // adjust parent indices of siblings to the right.
+        for i in (parent.idx + 1)..new_len {
+            unsafe {
+                let sibling = parent.node.edge_area_mut(i).assume_init_read();
+
+                (&raw mut (*sibling.as_ptr()).parent_idx).write(MaybeUninit::new(i as u16));
+            }
+        }
+
         unsafe { self.borrow_mut().dormant().awaken() }
     }
 }
@@ -1487,6 +1495,19 @@ where
         }
     }
 
+    pub fn as_subtree<'a>(&'a self) -> subtree::Subtree<'a, K, V, marker::Immut<'a>> {
+        match self.root.as_mut() {
+            Some(node) => {
+                let dormant = node.borrow_mut().dormant();
+                Subtree::new_root(unsafe { dormant.awaken().reborrow() })
+            }
+            None => {
+                let (_, dormant) = borrow::DormantMutRef::new(self);
+                Subtree::new_empty(dormant)
+            }
+        }
+    }
+
     pub fn entry<'a, Q>(&'a mut self, key_seq: Q) -> entry::Entry<'a, OnceAndIter<Q, K>, K, V>
     where
         Q: Iterator<Item = K>,
@@ -1771,6 +1792,7 @@ mod tests {
         tree.entry("asdf".chars()).or_insert(1);
         tree.entry("asd".chars()).or_insert(2);
         tree.entry("asdg".chars()).or_insert(3);
+        tree.entry("asda".chars()).or_insert(4);
 
         tree
     }
@@ -1779,4 +1801,11 @@ mod tests {
     fn drop_tree() {
         let tree = build_tree();
     }
+
+    #[test]
+    fn entry() {
+        let tree = build_tree();
+
+        assert_eq!(tree.as_subtree_mut().get("asdf".chars()), Some(&1));
+    }
 }