diff --git a/src/AcDream.App/UI/UiItemList.cs b/src/AcDream.App/UI/UiItemList.cs
index df77f47e..761d4db3 100644
--- a/src/AcDream.App/UI/UiItemList.cs
+++ b/src/AcDream.App/UI/UiItemList.cs
@@ -24,8 +24,14 @@ public sealed class UiItemList : UiElement
 
     public Func<uint, (uint tex, int w, int h)>? SpriteResolve { get; set; }
 
-    /// <summary>Convenience for single-cell slots (the toolbar): the first cell.</summary>
-    public UiItemSlot Cell => _cells[0];
+    /// <summary>Convenience for single-cell slots (the toolbar): the first cell.
+    /// Valid only while the list has at least one cell; after <see cref="Flush"/>
+    /// (the inventory-phase rebuild path) the list is empty until <see cref="AddItem"/>
+    /// runs, so use <see cref="GetItem"/> there instead.</summary>
+    /// <exception cref="InvalidOperationException">the list has no cells (e.g. after Flush).</exception>
+    public UiItemSlot Cell => _cells.Count > 0
+        ? _cells[0]
+        : throw new InvalidOperationException("UiItemList has no cells; call AddItem first or use GetItem(index).");
 
     public int GetNumUIItems() => _cells.Count;