diff --git a/src/logic/future_moves.rs b/src/logic/future_moves.rs
index 2e7288a..72548d1 100644
--- a/src/logic/future_moves.rs
+++ b/src/logic/future_moves.rs
@@ -308,15 +308,21 @@ impl FutureMoves {
 
         let mut index_map = vec![None; self.arena.len()];
 
-        self.arena = retain
+        let new_start: Vec<(usize, (usize, Move))> = retain
             .into_iter()
             .enumerate() // old_idx
             .zip(self.arena.drain(..))
             .flat_map(|((old_idx, keep), node)| keep.then_some((old_idx, node))) // filter out unrelated nodes
             .enumerate() // new_idx
-            .map(|(new_idx, (old_idx, mut node))| {
-                index_map[old_idx] = Some(new_idx);
+            .collect();
 
+        for &(new_idx, (old_idx, _)) in &new_start {
+            index_map[old_idx] = Some(new_idx);
+        }
+
+        self.arena = new_start
+            .into_iter()
+            .map(|(_, (_, mut node))| {
                 if let Some(parent) = node.parent.as_mut() {
                     if let Some(new_parent) = index_map[*parent] {
                         *parent = new_parent;
@@ -331,7 +337,7 @@ impl FutureMoves {
                         *c = new_c;
                         true
                     } else {
-                        false
+                        panic!("node is not kept in move");
                     }
                 });