diff --git a/src/LineageTree/lineageTree.py b/src/LineageTree/lineageTree.py
index e8629ca..38c8c75 100644
--- a/src/LineageTree/lineageTree.py
+++ b/src/LineageTree/lineageTree.py
@@ -965,7 +965,7 @@ def write(self, fname: str) -> None:
         fname : str
             path to and name of the file to save
         """
-        if os.path.splitext(fname)[-1] != ".lT":
+        if os.path.splitext(fname)[-1].upper() != ".LT":
             fname = os.path.extsep.join((fname, "lT"))
         if hasattr(self, "_protected_predecessor"):
             del self._protected_predecessor
diff --git a/src/LineageTree/lineageTreeManager.py b/src/LineageTree/lineageTreeManager.py
index 41c3677..89f7817 100644
--- a/src/LineageTree/lineageTreeManager.py
+++ b/src/LineageTree/lineageTreeManager.py
@@ -3,7 +3,7 @@
 import os
 import pickle as pkl
 import warnings
-from collections.abc import Callable, Iterable, Generator
+from collections.abc import Callable, Generator, Iterable
 from functools import partial
 from typing import TYPE_CHECKING, Literal
 
@@ -140,7 +140,7 @@ def write(self, fname: str):
         fname : str
             The path and name of the file that is to be saved.
         """
-        if os.path.splitext(fname)[-1] != ".lTM":
+        if os.path.splitext(fname)[-1].upper() != ".LTM":
             fname = os.path.extsep.join((fname, "lTM"))
         for _, lT in self:
             if hasattr(lT, "_protected_predecessor"):