diff --git a/src/calibre/gui2/viewer/bookmarks.py b/src/calibre/gui2/viewer/bookmarks.py
index bc918ada01..81146cb53a 100644
--- a/src/calibre/gui2/viewer/bookmarks.py
+++ b/src/calibre/gui2/viewer/bookmarks.py
@@ -10,7 +10,7 @@ from qt.core import (
     QPushButton, Qt, QWidget, pyqtSignal,
 )
 
-from calibre.gui2 import choose_files, choose_save_file
+from calibre.gui2 import choose_files, choose_save_file, error_dialog
 from calibre.gui2.dialogs.confirm_delete import confirm
 from calibre.gui2.gestures import GestureManager
 from calibre.gui2.viewer.shortcuts import get_shortcut_for
@@ -367,6 +367,9 @@ class BookmarkManager(QWidget):
             return
         title = self.uniqify_bookmark_title(title)
         cfi = (pos_data.get('selection_bounds') or {}).get('start') or pos_data['cfi']
+        if not cfi:
+            error_dialog(self, _('Failed to bookmark'), _('Could not calculate position in book'), show=True)
+            return
         bm = {
             'title': title,
             'pos_type': 'epubcfi',