diff --git a/cps/helper.py b/cps/helper.py
index 99c37e16..c427b483 100755
--- a/cps/helper.py
+++ b/cps/helper.py
@@ -75,11 +75,11 @@ except (ImportError, RuntimeError) as e:
 def convert_book_format(book_id, calibre_path, old_book_format, new_book_format, user_id, ereader_mail=None):
     book = calibre_db.get_book(book_id)
     data = calibre_db.get_book_format(book.id, old_book_format)
-    file_path = os.path.join(calibre_path, book.path, data.name)
     if not data:
         error_message = _(u"%(format)s format not found for book id: %(book)d", format=old_book_format, book=book_id)
         log.error("convert_book_format: %s", error_message)
         return error_message
+    file_path = os.path.join(calibre_path, book.path, data.name)
     if config.config_use_google_drive:
         if not gd.getFileFromEbooksFolder(book.path, data.name + "." + old_book_format.lower()):
             error_message = _(u"%(format)s not found on Google Drive: %(fn)s",
diff --git a/optional-requirements.txt b/optional-requirements.txt
index d18463a3..57a4038f 100644
--- a/optional-requirements.txt
+++ b/optional-requirements.txt
@@ -41,4 +41,4 @@ natsort>=2.2.0,<8.2.0
 comicapi>=2.2.0,<2.3.0
 
 # Kobo integration
-jsonschema>=3.2.0,<4.8.0
+jsonschema>=3.2.0,<4.9.0