diff --git a/fdfd_tools/waveguide_mode.py b/fdfd_tools/waveguide_mode.py
index aac718d..398f2eb 100644
--- a/fdfd_tools/waveguide_mode.py
+++ b/fdfd_tools/waveguide_mode.py
@@ -425,6 +425,8 @@ def compute_source_wg(E: field_t,
                          omega=omega, dxes=dxes, axis=axis,
                          polarity=polarity, slices=slices4,
                          epsilon=epsilon, mu=mu)
+    return J
+
 
 def compute_overlap_ce(E: field_t,
                        wavenumber: complex,