diff --git a/examples/Example212_PeriodicBoundary2D.jl b/examples/Example212_PeriodicBoundary2D.jl
index e853ad79..24bd7bd9 100644
--- a/examples/Example212_PeriodicBoundary2D.jl
+++ b/examples/Example212_PeriodicBoundary2D.jl
@@ -153,7 +153,7 @@ function main(;
             return nothing
         end
 
-        coupling_matrix = get_periodic_coupling_matrix(FES, xgrid, reg_left, reg_right, give_opposite!)
+        coupling_matrix = get_periodic_coupling_matrix(FES, reg_left, reg_right, give_opposite!)
         display(coupling_matrix)
         assign_operator!(PD, CombineDofs(u, u, coupling_matrix; kwargs...))
     end
diff --git a/examples/Example312_PeriodicBoundary3D.jl b/examples/Example312_PeriodicBoundary3D.jl
index 7e982702..671bc869 100644
--- a/examples/Example312_PeriodicBoundary3D.jl
+++ b/examples/Example312_PeriodicBoundary3D.jl
@@ -168,7 +168,7 @@ function main(;
             y[1] = width - x[1]
             return nothing
         end
-        coupling_matrix = get_periodic_coupling_matrix(FES, xgrid, reg_left, reg_right, give_opposite!)
+        coupling_matrix = get_periodic_coupling_matrix(FES, reg_left, reg_right, give_opposite!)
         display(coupling_matrix)
         assign_operator!(PD, CombineDofs(u, u, coupling_matrix; kwargs...))
     end
diff --git a/src/helper_functions.jl b/src/helper_functions.jl
index 193cf061..e5f14186 100644
--- a/src/helper_functions.jl
+++ b/src/helper_functions.jl
@@ -347,6 +347,10 @@ function _get_periodic_coupling_matrix(
                 fe_vector.entries[local_dof] = 1.0
 
                 # interpolate on the opposite boundary using x_trafo = give_opposite
+                if !haskey(search_areas, face_numbers_of_bfaces[i_boundary_face])
+                    throw("face $(face_numbers_of_bfaces[i_boundary_face]) is not on source boundary. Are the from/to regions and the give_opposite function correct?")
+                end
+
                 interpolate!(
                     fe_vector_target[1],
                     ON_FACES, eval_point,