diff --git a/src/enlyze/api_clients/production_runs/models.py b/src/enlyze/api_clients/production_runs/models.py
index 37bcfb0..192c2d3 100644
--- a/src/enlyze/api_clients/production_runs/models.py
+++ b/src/enlyze/api_clients/production_runs/models.py
@@ -47,7 +47,7 @@ def to_user_model(self) -> user_models.Product:
 
 
 class Quantity(ProductionRunsApiModel):
-    unit: str
+    unit: str | None
     value: float
 
     def to_user_model(self) -> user_models.Quantity:
diff --git a/src/enlyze/models.py b/src/enlyze/models.py
index 429965b..fa0b9d9 100644
--- a/src/enlyze/models.py
+++ b/src/enlyze/models.py
@@ -222,7 +222,7 @@ class Quantity:
     """Representation of a physical quantity"""
 
     #: Physical unit of quantity
-    unit: str
+    unit: str | None
 
     #: The quantity expressed in `unit`
     value: float