diff --git a/src/calibre/gui2/convert/regex_builder.py b/src/calibre/gui2/convert/regex_builder.py
index 3ce8ac36d5..f3c1f0a826 100644
--- a/src/calibre/gui2/convert/regex_builder.py
+++ b/src/calibre/gui2/convert/regex_builder.py
@@ -211,6 +211,9 @@ class RegexEdit(QWidget, Ui_Edit):
         self.button.clicked.connect(self.builder)
 
     def builder(self):
+        if self.db is None:
+            self.doc_cache = _('Click the Open button below to open a '
+                    'ebook to use for testing.')
         bld = RegexBuilder(self.db, self.book_id, self.edit.text(), self.doc_cache, self)
         if bld.cancelled:
             return