diff --git a/src/calibre/customize/zipplugin.py b/src/calibre/customize/zipplugin.py
index 02764ca09b..d7e5b759e9 100644
--- a/src/calibre/customize/zipplugin.py
+++ b/src/calibre/customize/zipplugin.py
@@ -248,7 +248,11 @@ class CalibrePluginFinder:
                     fullname_in_plugin += '.__init__'
                 else:
                     return
-        is_package = fullname.count('.') < 2 or fullname == '__init__' or fullname.endswith('__init__')
+        is_package = bool(
+            fullname.count('.') < 2 or
+            fullname_in_plugin == '__init__' or
+            (fullname_in_plugin and fullname_in_plugin.endswith('__init__'))
+        )
         if zip_file_path:
             filename = posixpath.join(zip_file_path, *fullname_in_plugin.split('.')) + '.py'