diff --git a/dan/datasets/extract/__init__.py b/dan/datasets/extract/__init__.py
index 880c0022c96d85d98ca0f258f880a8e11cca3136..d04412f89d48fe3fc7fc5ca740d676f490c9e9d8 100644
--- a/dan/datasets/extract/__init__.py
+++ b/dan/datasets/extract/__init__.py
@@ -39,7 +39,7 @@ def validate_char(char):
 def _valid_image_format(value: str):
     im_format = value
     if not im_format.startswith("."):
-        im_format.insert(0, ".")
+        im_format = "." + im_format
     return im_format