diff --git a/src/nomad_material_processing/general.py b/src/nomad_material_processing/general.py
index 645444a..e4e0f02 100644
--- a/src/nomad_material_processing/general.py
+++ b/src/nomad_material_processing/general.py
@@ -968,7 +968,7 @@ def normalize(self, archive: 'EntryArchive', logger: 'BoundLogger') -> None:
             logger (BoundLogger): A structlog logger.
         """
         self.components = []
-        if self.substrate.reference:
+        if self.substrate and self.substrate.reference:
             self.components.append(
                 SystemComponent(
                     name=self.substrate.name, system=self.substrate.reference