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 2315e53220b..3dc973fba39 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
@@ -194,6 +194,8 @@ def _determine_extension(text, fetcher):
                 src_format = 'setuptools'
             elif src_format == 'flit':
                 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.")
             extension = FORMATS[src_format]
 
     elif fetcher == 'fetchurl':
@@ -326,4 +328,4 @@ def main():
 
 
 if __name__ == '__main__':
-    main()
\ No newline at end of file
+    main()