diff --git a/source/RevitLookup/Core/ComponentModel/Descriptors/DocumentDescriptor.cs b/source/RevitLookup/Core/ComponentModel/Descriptors/DocumentDescriptor.cs
index 23e9d8ab..fb024b00 100644
--- a/source/RevitLookup/Core/ComponentModel/Descriptors/DocumentDescriptor.cs
+++ b/source/RevitLookup/Core/ComponentModel/Descriptors/DocumentDescriptor.cs
@@ -24,7 +24,7 @@
 
 namespace RevitLookup.Core.ComponentModel.Descriptors;
 
-public sealed class DocumentDescriptor : Descriptor, IDescriptorResolver
+public sealed class DocumentDescriptor : Descriptor, IDescriptorResolver, IDescriptorExtension
 {
     private readonly Document _document;
     
@@ -71,4 +71,14 @@ IVariants ResolveGetAllUnusedElements()
         }
 #endif
     }
+    
+    public void RegisterExtensions(IExtensionManager manager)
+    {
+        if (!_document.IsFamilyDocument) return;
+        manager.Register(nameof(FamilySizeTableManager.GetFamilySizeTableManager), context =>
+        {
+            var result = FamilySizeTableManager.GetFamilySizeTableManager(context, new ElementId(BuiltInParameter.RBS_LOOKUP_TABLE_NAME));
+            return Variants.Single(result);
+        });
+    }
 }
\ No newline at end of file