diff --git a/gprMax/cmds_geometry/add_surface_roughness.py b/gprMax/cmds_geometry/add_surface_roughness.py
index 00eb86cf..a79898a5 100644
--- a/gprMax/cmds_geometry/add_surface_roughness.py
+++ b/gprMax/cmds_geometry/add_surface_roughness.py
@@ -154,7 +154,7 @@ class AddSurfaceRoughness(UserObjectGeometry):
             if zs == zf:
                 logger.exception(f"{self.__str__()} dimensions are not specified correctly")
                 raise ValueError
-            if ys not in [volume.ys and volume.yf]:
+            if ys not in [volume.ys, volume.yf]:
                 logger.exception(f"{self.__str__()} can only be used on the external " + "surfaces of a fractal box")
                 raise ValueError
             fractalrange = (round_value(limits[0] / grid.dy), round_value(limits[1] / grid.dy))
@@ -182,7 +182,7 @@ class AddSurfaceRoughness(UserObjectGeometry):
                 requestedsurface = "yplus"
 
         elif zs == zf:
-            if zs not in [volume.zs and volume.zf]:
+            if zs not in [volume.zs, volume.zf]:
                 logger.exception(f"{self.__str__()} can only be used on the external " + "surfaces of a fractal box")
                 raise ValueError
             fractalrange = (round_value(limits[0] / grid.dz), round_value(limits[1] / grid.dz))
diff --git a/gprMax/cuda_opencl/knl_fields_updates.py b/gprMax/cuda_opencl/knl_fields_updates.py
index 70012904..91fc8293 100644
--- a/gprMax/cuda_opencl/knl_fields_updates.py
+++ b/gprMax/cuda_opencl/knl_fields_updates.py
@@ -222,7 +222,8 @@ update_electric_dispersive_A = {
     //                                              dispersive, ID and field
     //                                              component arrays.
 
-
+    $CUDA_IDX
+    
     // Convert the linear index to subscripts for 3D field arrays
     int x = i / ($NY_FIELDS * $NZ_FIELDS);
     int y = (i % ($NY_FIELDS * $NZ_FIELDS)) / $NZ_FIELDS;
@@ -238,7 +239,6 @@ update_electric_dispersive_A = {
     int y_T = ((i % ($NX_T * $NY_T * $NZ_T)) % ($NY_T * $NZ_T)) / $NZ_T;
     int z_T = ((i % ($NX_T * $NY_T * $NZ_T)) % ($NY_T * $NZ_T)) % $NZ_T;
 
-    $CUDA_IDX
 
     // Ex component
     if ((NY != 1 || NZ != 1) && x >= 0 && x < NX && y > 0 && y < NY && z > 0 && z < NZ) {
@@ -334,7 +334,8 @@ update_electric_dispersive_B = {
     //                                              dispersive, ID and field
     //                                              component arrays.
 
-
+    $CUDA_IDX
+        
     // Convert the linear index to subscripts for 3D field arrays
     int x = i / ($NY_FIELDS * $NZ_FIELDS);
     int y = (i % ($NY_FIELDS * $NZ_FIELDS)) / $NZ_FIELDS;
@@ -350,7 +351,6 @@ update_electric_dispersive_B = {
     int y_T = ((i % ($NX_T * $NY_T * $NZ_T)) % ($NY_T * $NZ_T)) / $NZ_T;
     int z_T = ((i % ($NX_T * $NY_T * $NZ_T)) % ($NY_T * $NZ_T)) % $NZ_T;
 
-    $CUDA_IDX
 
     // Ex component
     if ((NY != 1 || NZ != 1) && x >= 0 && x < NX && y > 0 && y < NY && z > 0 && z < NZ) {