diff --git a/product/ERP5Form/ListBox.py b/product/ERP5Form/ListBox.py
index 25f2611ff157464b7f9bbbbe3f312e1fc125d133..3f82e4aa8768897ff88494b22dab8f639c7112e9 100644
--- a/product/ERP5Form/ListBox.py
+++ b/product/ERP5Form/ListBox.py
@@ -2154,7 +2154,7 @@ class ListBoxHTMLRenderer(ListBoxRenderer):
     list_lines =self.getParamDict().get('list_lines', None)
     if list_lines is not None:
       # it's possible to override max lines from selection parameters
-      return list_lines 
+      return int(list_lines)
     return self.field.get_value('lines')
 
   getMaxLineNumber = VolatileCachingMethod(getMaxLineNumber)