diff --git a/src/robofish/io/file.py b/src/robofish/io/file.py
index d6ce9bd2276ab6f7f303e491d24a3548179f1033..1ce2f1ee4e3119ae178919e0f7620b80f41ec2db 100644
--- a/src/robofish/io/file.py
+++ b/src/robofish/io/file.py
@@ -67,7 +67,7 @@ class File(h5py.File):
         mode: str = "r",
         *,  # PEP 3102
         world_size_cm: List[int] = None,
-        world_shape: str = None,
+        world_shape: str = "rectangle",
         validate: bool = False,
         validate_when_saving: bool = True,
         strict_validate: bool = False,