diff --git a/cura/QualityManager.py b/cura/QualityManager.py
index fb3a623466..4f57bc1a8d 100644
--- a/cura/QualityManager.py
+++ b/cura/QualityManager.py
@@ -159,7 +159,8 @@ class QualityManager:
         else:
             material_ids = set()
             for material_instance in material_containers:
-                material_ids.add(material_instance.getId())
+                if material_instance is not None:
+                    material_ids.add(material_instance.getId())
 
         containers = UM.Settings.ContainerRegistry.getInstance().findInstanceContainers(**criteria)