diff --git a/src/calibre/gui2/convert/toc.ui b/src/calibre/gui2/convert/toc.ui
index 6140816804..0e7b6530ea 100644
--- a/src/calibre/gui2/convert/toc.ui
+++ b/src/calibre/gui2/convert/toc.ui
@@ -15,7 +15,11 @@
   </property>
   <layout class="QGridLayout" name="gridLayout">
    <item row="7" column="1">
-    <widget class="QSpinBox" name="opt_toc_threshold"/>
+    <widget class="QSpinBox" name="opt_toc_threshold">
+     <property name="maximum">
+      <number>10000</number>
+     </property>
+    </widget>
    </item>
    <item row="1" column="0" colspan="2">
     <widget class="QCheckBox" name="opt_use_auto_toc">