diff --git a/tardis/interfaces/siteadapter.py b/tardis/interfaces/siteadapter.py
index 68f5e406..dd82bb1c 100644
--- a/tardis/interfaces/siteadapter.py
+++ b/tardis/interfaces/siteadapter.py
@@ -20,7 +20,7 @@ class SiteAdapterBaseModel(BaseModel):
     """
 
     MachineTypes: List[str]
-    MachineTypeConfiguration: "AttributeDict[str, AttributeDict[str, Any]]"
+    MachineTypeConfiguration: "AttributeDict[str, Optional[AttributeDict[str, Any]]]"
     MachineMetaData: "AttributeDict[str, AttributeDict[str, Any]]"
     # Use Any to avoid automated conversion to int or float here, validate later