diff --git a/client/src/components/rack/RackPlanner.tsx b/client/src/components/rack/RackPlanner.tsx
index 119738f..f22fb61 100644
--- a/client/src/components/rack/RackPlanner.tsx
+++ b/client/src/components/rack/RackPlanner.tsx
@@ -69,13 +69,17 @@ function ModuleDragOverlay({ label }: { label: string }) {
  *    (which needs the sortable rack targets) still works.
  */
 const slotFirstCollision: CollisionDetection = (args) => {
-  // Restrict to slot droppables only
-  const slotContainers = args.droppableContainers.filter(
+  // droppableContainers is a custom NodeMap (not a plain Array) — it only
+  // implements [Symbol.iterator], so .filter() doesn't exist on it.
+  // Convert to Array first before filtering.
+  const allContainers = Array.from(args.droppableContainers);
+
+  const slotContainers = allContainers.filter(
     (c) => c.data.current?.dropType === 'slot'
   );
 
   if (slotContainers.length > 0) {
-    const slotHits = pointerWithin({ ...args, droppableContainers: slotContainers });
+    const slotHits = pointerWithin({ ...args, droppableContainers: slotContainers as typeof args.droppableContainers });
     if (slotHits.length > 0) return slotHits;
   }