diff --git a/modal/network_file_system.py b/modal/network_file_system.py
index ddf46c832..4a0e753f1 100644
--- a/modal/network_file_system.py
+++ b/modal/network_file_system.py
@@ -243,6 +243,8 @@ async def write_file(self, remote_path: str, fp: BinaryIO, progress_cb: Callable
         If remote_path ends with `/` it's assumed to be a directory and the
         file will be uploaded with its current name to that directory.
         """
+        progress_cb = progress_cb or (lambda *_, **__: None)
+
         sha_hash = get_sha256_hex(fp)
         fp.seek(0, os.SEEK_END)
         data_size = fp.tell()