diff --git a/cura/Machines/MaterialManager.py b/cura/Machines/MaterialManager.py
index 2cdc784a5e..24d4e4ec38 100644
--- a/cura/Machines/MaterialManager.py
+++ b/cura/Machines/MaterialManager.py
@@ -236,7 +236,7 @@ class MaterialManager(QObject):
         return self._material_group_map.get(root_material_id)
 
     def getRootMaterialIDForDiameter(self, root_material_id: str, approximate_diameter: str) -> str:
-        return self._material_diameter_map.get(root_material_id).get(approximate_diameter, root_material_id)
+        return self._material_diameter_map.get(root_material_id, {}).get(approximate_diameter, root_material_id)
 
     def getRootMaterialIDWithoutDiameter(self, root_material_id: str) -> str:
         return self._diameter_material_map.get(root_material_id)