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))