diff --git a/PathPlanning/Dijkstra/dijkstra.py b/PathPlanning/Dijkstra/dijkstra.py
index f5a4703910..2fee6e928d 100644
--- a/PathPlanning/Dijkstra/dijkstra.py
+++ b/PathPlanning/Dijkstra/dijkstra.py
@@ -143,7 +143,7 @@ def calc_xy_index(self, position, minp):
         return round((position - minp) / self.resolution)
 
     def calc_index(self, node):
-        return (node.y - self.min_y) * self.x_width + (node.x - self.min_x)
+        return node.y * self.x_width + node.x
 
     def verify_node(self, node):
         px = self.calc_position(node.x, self.min_x)
diff --git a/tests/test_dijkstra.py b/tests/test_dijkstra.py
index 40404153d8..4f6d3229e2 100644
--- a/tests/test_dijkstra.py
+++ b/tests/test_dijkstra.py
@@ -7,5 +7,38 @@ def test_1():
     m.main()
 
 
+def test_calc_index_uses_grid_coordinates():
+    # Grid indices are relative to the map origin and measured in cells.
+    for min_x, min_y, resolution in ((-4.0, 3.0, 0.5), (6.0, -5.0, 2.0)):
+        ox = [min_x, min_x + 10.0, min_x, min_x + 10.0]
+        oy = [min_y, min_y, min_y + 10.0, min_y + 10.0]
+        planner = m.DijkstraPlanner(ox, oy, resolution, robot_radius=0.0)
+
+        assert planner.calc_xy_index(min_x + 2 * resolution, min_x) == 2
+        assert planner.calc_xy_index(min_y + 3 * resolution, min_y) == 3
+        assert planner.calc_index(planner.Node(0, 0, 0.0, -1)) == 0
+        assert planner.calc_index(planner.Node(2, 3, 0.0, -1)) == (
+            3 * planner.x_width + 2
+        )
+        assert planner.calc_index(
+            planner.Node(planner.x_width - 1, planner.y_width - 1, 0.0, -1)
+        ) == planner.x_width * planner.y_width - 1
+
+
+
+def test_planning_preserves_start_when_grid_key_would_be_negative_one():
+    # The old index formula gave cell (0, 1) key -1, which is also the
+    # parent-chain sentinel, so path reconstruction stopped before the start.
+    m.show_animation = False
+    planner = m.DijkstraPlanner(
+        [1.0, 5.0, 1.0, 5.0], [1.0, 1.0, 5.0, 5.0],
+        1.0, 0.0
+    )
+
+    rx, ry = planner.planning(1.0, 2.0, 1.0, 3.0)
+
+    assert list(zip(rx, ry)) == [(1.0, 3.0), (1.0, 2.0)]
+
+
 if __name__ == '__main__':
     conftest.run_this_test(__file__)