diff --git a/src/molearn/data/pdb_data.py b/src/molearn/data/pdb_data.py
index 1d25074..59bab91 100644
--- a/src/molearn/data/pdb_data.py
+++ b/src/molearn/data/pdb_data.py
@@ -59,7 +59,7 @@ def __init__(self, filename=None, topology=None, fix_terminal=False, atoms=None)
             if atoms is not None:
                 self.atomselect(atoms=atoms)
 
-    def import_pdb(self, filename: str | list[str], topology: str | None):
+    def import_pdb(self, filename: str | list[str], topology: str | None = None):
         """
         Load one or multiple trajectory files