diff --git a/src/calibre/ebooks/oeb/polish/create.py b/src/calibre/ebooks/oeb/polish/create.py
index e55ec97962..982a4e97b2 100644
--- a/src/calibre/ebooks/oeb/polish/create.py
+++ b/src/calibre/ebooks/oeb/polish/create.py
@@ -24,6 +24,8 @@ from calibre.utils.localization import lang_as_iso639_1
 from calibre.utils.logging import DevNull
 from calibre.utils.zipfile import ZipFile, ZIP_STORED
 
+valid_empty_formats = {'epub', 'txt', 'docx', 'azw3'}
+
 def create_toc(mi, opf, html_name, lang):
     uuid = ''
     for u in opf.xpath('//*[@id="uuid_id"]'):
@@ -34,7 +36,9 @@ def create_toc(mi, opf, html_name, lang):
 
 def create_book(mi, path, fmt='epub', opf_name='metadata.opf', html_name='start.xhtml', toc_name='toc.ncx'):
     ''' Create an empty book in the specified format at the specified location. '''
-    if fmt not in ['epub', 'azw3']:
+    if fmt not in valid_empty_formats:
+        raise ValueError('Cannot create empty book in the %s format' % fmt)
+    if fmt not in {'epub', 'azw3'}:
         return
     path = os.path.abspath(path)
     lang = 'und'
diff --git a/src/calibre/gui2/actions/add.py b/src/calibre/gui2/actions/add.py
index 94477f5df4..022076ee6b 100644
--- a/src/calibre/gui2/actions/add.py
+++ b/src/calibre/gui2/actions/add.py
@@ -14,7 +14,7 @@ from PyQt5.Qt import QPixmap, QTimer
 from calibre import as_unicode
 from calibre.gui2 import (error_dialog, choose_files, choose_dir,
         warning_dialog, info_dialog, gprefs)
-from calibre.gui2.dialogs.add_empty_book import AddEmptyBookDialog, valid_empty_formats
+from calibre.gui2.dialogs.add_empty_book import AddEmptyBookDialog
 from calibre.gui2.dialogs.confirm_delete import confirm
 from calibre.gui2.dialogs.progress import ProgressDialog
 from calibre.gui2.widgets import IMAGE_EXTENSIONS
@@ -79,6 +79,7 @@ class AddAction(InterfaceAction):
         ma('add-formats', _('Add files to selected book records'),
                 triggered=self.add_formats, shortcut='Shift+A')
         arm = self.add_archive_menu = self.add_menu.addMenu(_('Add an empty file to selected book records'))
+        from calibre.ebooks.oeb.polish.create import valid_empty_formats
         for fmt in sorted(valid_empty_formats):
             self.create_menu_action(arm, 'add-empty-' + fmt,
                                     _('Add empty {}').format(fmt.upper())).triggered.connect(
diff --git a/src/calibre/gui2/dialogs/add_empty_book.py b/src/calibre/gui2/dialogs/add_empty_book.py
index c7dffadb0f..225dcabb60 100644
--- a/src/calibre/gui2/dialogs/add_empty_book.py
+++ b/src/calibre/gui2/dialogs/add_empty_book.py
@@ -12,7 +12,6 @@ from calibre.gui2.complete2 import EditWithComplete
 from calibre.utils.config import tweaks
 from calibre.gui2 import gprefs
 
-valid_empty_formats = {'epub', 'txt', 'docx', 'azw3'}
 
 class AddEmptyBookDialog(QDialog):
 
@@ -81,6 +80,7 @@ class AddEmptyBookDialog(QDialog):
         self.format_label = QLabel(_('Also create an empty ebook in format:'))
         self._layout.addWidget(self.format_label, 8, 0, 1, 2)
         c = self.format_value = QComboBox(self)
+        from calibre.ebooks.oeb.polish.create import valid_empty_formats
         possible_formats = [''] + sorted(x.upper() for x in valid_empty_formats)
         c.addItems(possible_formats)
         c.setToolTip(_('Also create an empty book format file that you can subsequently edit'))