diff --git a/src/pyj/read_book/db.pyj b/src/pyj/read_book/db.pyj
index dc2dddd352..38f8800150 100644
--- a/src/pyj/read_book/db.pyj
+++ b/src/pyj/read_book/db.pyj
@@ -42,7 +42,7 @@ def get_error_details(event):
             desc = _('{}: Try deleting some stored books first').format('QuotaExceededError')
         elif desc.name:
             desc = desc.name
-    return desc or ''
+    return desc or 'Unknown Error'
 
 DB_NAME = 'calibre'
 DB_VERSION = 1