diff --git a/pkgs/development/interpreters/python/update-python-libraries/update-python-libraries.py b/pkgs/development/interpreters/python/update-python-libraries/update-python-libraries.py
index 82b2aac39a9f..7e0580ce063d 100755
--- a/pkgs/development/interpreters/python/update-python-libraries/update-python-libraries.py
+++ b/pkgs/development/interpreters/python/update-python-libraries/update-python-libraries.py
@@ -229,6 +229,8 @@ def _determine_extension(text, fetcher):
                 raise ValueError("Don't know how to update a Flit package.")
             elif src_format == 'other':
                 raise ValueError("Don't know how to update a format='other' package.")
+            elif src_format == 'pyproject':
+                raise ValueError("Don't know how to update a pyproject package.")
             extension = FORMATS[src_format]
 
     elif fetcher == 'fetchurl':