diff --git a/src/calibre/gui2/viewer/ui.py b/src/calibre/gui2/viewer/ui.py
index 5a7c953795..edabdabd2a 100644
--- a/src/calibre/gui2/viewer/ui.py
+++ b/src/calibre/gui2/viewer/ui.py
@@ -444,7 +444,10 @@ class EbookViewer(MainWindow):
             merge_annotations(parse_annotations(raw), amap)
 
     def update_window_title(self):
-        title = self.current_book_data['metadata']['title']
+        try:
+            title = self.current_book_data['metadata']['title']
+        except Exception:
+            title = _('Unknown')
         book_format = self.current_book_data['manifest']['book_format']
         title = '{} [{}] — {}'.format(title, book_format, self.base_window_title)
         self.setWindowTitle(title)
diff --git a/src/pyj/read_book/open_book.pyj b/src/pyj/read_book/open_book.pyj
index 17b86a904e..2b10fc333e 100644
--- a/src/pyj/read_book/open_book.pyj
+++ b/src/pyj/read_book/open_book.pyj
@@ -32,7 +32,7 @@ def create_open_book(container, book):
             fname = entry.pathtoebook.replace(/\\/g, '/')
             if '/' in fname:
                 fname = fname.rpartition('/')[-1]
-            items.push(create_item(entry.title, ui_operations.ask_for_open.bind(None, entry.pathtoebook), fname))
+            items.push(create_item(entry.title or _('Unknown'), ui_operations.ask_for_open.bind(None, entry.pathtoebook), fname))
         create_item_list(c.lastChild, items)
 
         c.appendChild(E.div(style='margin: 1rem'))
diff --git a/src/pyj/read_book/view.pyj b/src/pyj/read_book/view.pyj
index c104e8bf00..bba43543c7 100644
--- a/src/pyj/read_book/view.pyj
+++ b/src/pyj/read_book/view.pyj
@@ -575,7 +575,7 @@ class View:
         self.overlay.show_loading_message(msg)
 
     def show_loading(self):
-        msg = _('Loading next section from <i>{title}</i>, please wait…').format(title=self.book.metadata.title)
+        msg = _('Loading next section from <i>{title}</i>, please wait…').format(title=self.book.metadata.title or _('Unknown'))
         if self.show_loading_callback_timer is not None:
             clearTimeout(self.show_loading_callback_timer)
         self.show_loading_callback_timer = setTimeout(self.show_loading_message.bind(None, msg), 200)