diff --git a/meanas/fdfd/waveguide_mode.py b/meanas/fdfd/waveguide_mode.py
index 410ee02..35439a0 100644
--- a/meanas/fdfd/waveguide_mode.py
+++ b/meanas/fdfd/waveguide_mode.py
@@ -186,6 +186,7 @@ def compute_overlap_e(E: field_t,
                       axis: int,
                       polarity: int,
                       slices: List[slice],
+                      epsilon: field_t,         # TODO unused??
                       mu: field_t = None,
                       ) -> field_t:
     """