diff --git a/tardis/model/parse_input.py b/tardis/model/parse_input.py
index 738de1d08f2..10b359540be 100644
--- a/tardis/model/parse_input.py
+++ b/tardis/model/parse_input.py
@@ -732,7 +732,10 @@ def calculate_geometric_dilution_factor(geometry):
         1
         - np.sqrt(
             1
-            - (geometry.r_inner_active[0] ** 2 / geometry.r_middle_active**2)
+            - (
+                geometry.r_inner[geometry.v_inner_boundary_index] ** 2
+                / geometry.r_middle_active**2
+            )
             .to(1)
             .value
         )