diff --git a/gprMax/updates/updates.py b/gprMax/updates/updates.py
index 18a3868b..d1a2563f 100644
--- a/gprMax/updates/updates.py
+++ b/gprMax/updates/updates.py
@@ -18,13 +18,11 @@
 
 
 from abc import ABC, abstractmethod
-from typing import Generic
-
-from typing_extensions import TypeVar
+from typing import Generic, TypeVar
 
 from gprMax.grid.fdtd_grid import FDTDGrid
 
-GridType = TypeVar("GridType", bound=FDTDGrid, default=FDTDGrid)
+GridType = TypeVar("GridType", bound=FDTDGrid)
 
 
 class Updates(Generic[GridType], ABC):