diff --git a/meanas/fdtd/boundaries.py b/meanas/fdtd/boundaries.py
index 131d741..aa0bff5 100644
--- a/meanas/fdtd/boundaries.py
+++ b/meanas/fdtd/boundaries.py
@@ -28,17 +28,19 @@ def conducting_boundary(
         shifted1_slice = [slice(None)] * 3
         boundary_slice[direction] = 0
         shifted1_slice[direction] = 1
+        boundary = tuple(boundary_slice)
+        shifted1 = tuple(shifted1_slice)
 
         def en(e: fdfield_t) -> fdfield_t:
-            e[direction][boundary_slice] = 0
-            e[u][boundary_slice] = e[u][shifted1_slice]
-            e[v][boundary_slice] = e[v][shifted1_slice]
+            e[direction][boundary] = 0
+            e[u][boundary] = e[u][shifted1]
+            e[v][boundary] = e[v][shifted1]
             return e
 
         def hn(h: fdfield_t) -> fdfield_t:
-            h[direction][boundary_slice] = h[direction][shifted1_slice]
-            h[u][boundary_slice] = 0
-            h[v][boundary_slice] = 0
+            h[direction][boundary] = h[direction][shifted1]
+            h[u][boundary] = 0
+            h[v][boundary] = 0
             return h
 
         return en, hn
@@ -50,20 +52,23 @@ def conducting_boundary(
         boundary_slice[direction] = -1
         shifted1_slice[direction] = -2
         shifted2_slice[direction] = -3
+        boundary = tuple(boundary_slice)
+        shifted1 = tuple(shifted1_slice)
+        shifted2 = tuple(shifted2_slice)
 
         def ep(e: fdfield_t) -> fdfield_t:
-            e[direction][boundary_slice] = -e[direction][shifted2_slice]
-            e[direction][shifted1_slice] = 0
-            e[u][boundary_slice] = e[u][shifted1_slice]
-            e[v][boundary_slice] = e[v][shifted1_slice]
+            e[direction][boundary] = -e[direction][shifted2]
+            e[direction][shifted1] = 0
+            e[u][boundary] = e[u][shifted1]
+            e[v][boundary] = e[v][shifted1]
             return e
 
         def hp(h: fdfield_t) -> fdfield_t:
-            h[direction][boundary_slice] = h[direction][shifted1_slice]
-            h[u][boundary_slice] = -h[u][shifted2_slice]
-            h[u][shifted1_slice] = 0
-            h[v][boundary_slice] = -h[v][shifted2_slice]
-            h[v][shifted1_slice] = 0
+            h[direction][boundary] = h[direction][shifted1]
+            h[u][boundary] = -h[u][shifted2]
+            h[u][shifted1] = 0
+            h[v][boundary] = -h[v][shifted2]
+            h[v][shifted1] = 0
             return h
 
         return ep, hp