diff --git a/src/calibre/gui2/tweak_book/boss.py b/src/calibre/gui2/tweak_book/boss.py
index e7a466304b..c21e8dd0af 100644
--- a/src/calibre/gui2/tweak_book/boss.py
+++ b/src/calibre/gui2/tweak_book/boss.py
@@ -554,6 +554,8 @@ class Boss(QObject):
                 show_font_face_rule_for_font_files(c, added_fonts, self.gui)
 
     def add_cover(self):
+        if not self.ensure_book():
+            return
         d = AddCover(current_container(), self.gui)
         d.import_requested.connect(self.do_add_file)
         try:
@@ -567,7 +569,8 @@ class Boss(QObject):
         finally:
             d.import_requested.disconnect()
 
-    def ensure_book(self, msg):
+    def ensure_book(self, msg=None):
+        msg = msg or _('No book is currently open. You must first open a book.')
         if current_container() is None:
             error_dialog(self.gui, _('No book open'), msg, show=True)
             return False
@@ -601,6 +604,8 @@ class Boss(QObject):
         self.edit_file(name, 'html')
 
     def polish(self, action, name, parent=None):
+        if not self.ensure_book():
+            return
         from calibre.gui2.tweak_book.polish import get_customization, show_report
         customization = get_customization(action, name, parent or self.gui)
         if customization is None:
@@ -682,6 +687,8 @@ class Boss(QObject):
     # Renaming {{{
 
     def rationalize_folders(self):
+        if not self.ensure_book():
+            return
         c = current_container()
         if not c.SUPPORTS_FILENAMES:
             return error_dialog(self.gui, _('Not supported'),
@@ -848,6 +855,8 @@ class Boss(QObject):
             self.show_current_diff(allow_revert=allow_revert, to_container=to_container)
 
     def compare_book(self):
+        if not self.ensure_book():
+            return
         self.commit_all_editors_to_container()
         c = current_container()
         path = choose_files(self.gui, 'select-book-for-comparison', _('Choose book'), filters=[
@@ -975,6 +984,8 @@ class Boss(QObject):
                     'No class {} found to change').format(class_name), show=True)
 
     def set_semantics(self):
+        if not self.ensure_book():
+            return
         self.commit_all_editors_to_container()
         c = current_container()
         if c.book_type == 'azw3':