diff --git a/meanas/fdfd/waveguide_2d.py b/meanas/fdfd/waveguide_2d.py
index 00d33a0..cb9c12c 100644
--- a/meanas/fdfd/waveguide_2d.py
+++ b/meanas/fdfd/waveguide_2d.py
@@ -826,7 +826,7 @@ def sensitivity(
 
 
 def solve_modes(
-        mode_numbers: list[int],
+        mode_numbers: Sequence[int],
         omega: complex,
         dxes: dx_lists_t,
         epsilon: vfdfield_t,