diff --git a/lib/microwaveprop/workers/propagation_grid_worker.ex b/lib/microwaveprop/workers/propagation_grid_worker.ex
index be372347..2b733426 100644
--- a/lib/microwaveprop/workers/propagation_grid_worker.ex
+++ b/lib/microwaveprop/workers/propagation_grid_worker.ex
@@ -375,10 +375,12 @@ defmodule Microwaveprop.Workers.PropagationGridWorker do
     compute_scores_algorithm(grid_data, valid_time, forecast_hour == 0)
   end
 
-  defp compute_scores_algorithm(grid_data, valid_time, include_factors?) do
+  @doc false
+  # Public for testing. grid_data is a %{{lat, lon} => profile} map.
+  def compute_scores_algorithm(grid_data, valid_time, include_factors?) do
     Microwaveprop.Instrument.span(
       [:propagation_grid, :score_band],
-      %{point_count: length(grid_data)},
+      %{point_count: map_size(grid_data)},
       fn ->
         grid_data
         |> Task.async_stream(
diff --git a/test/microwaveprop/workers/propagation_grid_worker_test.exs b/test/microwaveprop/workers/propagation_grid_worker_test.exs
index 7286f82b..0a2febb4 100644
--- a/test/microwaveprop/workers/propagation_grid_worker_test.exs
+++ b/test/microwaveprop/workers/propagation_grid_worker_test.exs
@@ -29,6 +29,21 @@ defmodule Microwaveprop.Workers.PropagationGridWorkerTest do
     end
   end
 
+  describe "compute_scores_algorithm/3" do
+    test "accepts a map of {{lat, lon} => profile} without raising" do
+      # grid_data is a map keyed by {lat, lon}, not a list.
+      # A regression guard against using length/1 on the map.
+      valid_time = ~U[2026-04-19 15:00:00Z]
+
+      assert [] =
+               PropagationGridWorker.compute_scores_algorithm(
+                 %{},
+                 valid_time,
+                 false
+               )
+    end
+  end
+
   describe "perform/1 — chain seeding (empty args)" do
     test "enqueues all 19 forecast-hour jobs at once, all pointing at the same run_time" do
       Oban.Testing.with_testing_mode(:manual, fn ->