diff --git a/src/calibre/gui2/tweak_book/boss.py b/src/calibre/gui2/tweak_book/boss.py
index 616edf546a..9542ceb7ae 100644
--- a/src/calibre/gui2/tweak_book/boss.py
+++ b/src/calibre/gui2/tweak_book/boss.py
@@ -504,6 +504,8 @@ class Boss(QObject):
 
         def no_replace(prefix=''):
             QApplication.restoreOverrideCursor()
+            if prefix:
+                prefix += ' '
             error_dialog(
                 self.gui, _('Cannot replace'), prefix + _(
                 'You must first click Find, before trying to replace'), show=True)