diff --git a/boxtree/area_query.py b/boxtree/area_query.py
index 88cc4b625b233c9c5b5fcc7517929f3ee38e242b..a5ebdca20f178531bbe0fab095f322ceaf0941f6 100644
--- a/boxtree/area_query.py
+++ b/boxtree/area_query.py
@@ -835,16 +835,24 @@ class LeavesToBallsLookupBuilder(object):
 
         logger.info("leaves-to-balls lookup: expand starts")
 
-        nkeys = len(area_query.leaves_near_ball_lists)
+        nkeys = tree.nboxes
         nballs_p_1 = len(area_query.leaves_near_ball_starts)
         assert nballs_p_1 == len(ball_radii) + 1
 
+        # We invert the area query in two steps:
+        #
+        # 1. Turn the area query result into (ball number, box number) pairs.
+        #    This is done in the "starts expander kernel."
+        #
+        # 2. Key-value sort the (ball number, box number) pairs by box number.
+
         starts_expander_knl = self.get_starts_expander_kernel(tree.box_id_dtype)
-        expanded_starts = cl.array.empty(queue, nkeys, tree.box_id_dtype)
+        expanded_starts = cl.array.empty(
+                queue, len(area_query.leaves_near_ball_lists), tree.box_id_dtype)
         evt = starts_expander_knl(
-            expanded_starts,
-            area_query.leaves_near_ball_starts.with_queue(queue),
-            nballs_p_1)
+                expanded_starts,
+                area_query.leaves_near_ball_starts.with_queue(queue),
+                nballs_p_1)
         wait_for = [evt]
 
         logger.info("leaves-to-balls lookup: key-value sort")
diff --git a/test/test_tree.py b/test/test_tree.py
index 9fc08334b5a2f35bb2b9f66893accfb0a60374d8..0857787724a13cc43535d41cba7b713c0c6c16f5 100644
--- a/test/test_tree.py
+++ b/test/test_tree.py
@@ -636,6 +636,8 @@ def test_leaves_to_balls_query(ctx_getter, dims, do_plot=False):
     ball_centers = np.array([x.get() for x in ball_centers]).T
     ball_radii = ball_radii.get()
 
+    assert len(lbl.balls_near_box_starts) == tree.nboxes + 1
+
     from boxtree import box_flags_enum
 
     for ibox in range(tree.nboxes):