diff --git a/gprMax/geometry_outputs.py b/gprMax/geometry_outputs.py
index dbe16a84..71780cfa 100644
--- a/gprMax/geometry_outputs.py
+++ b/gprMax/geometry_outputs.py
@@ -341,7 +341,7 @@ class Comments():
 
     def materials_comment(self):
         if not self.averaged_materials:
-            return [m.ID for m in self.grid.materials if m.type is not 'dielectric-smoothed']
+            return [m.ID for m in self.grid.materials if m.type != 'dielectric-smoothed']
         else:
             return [m.ID for m in self.grid.materials]