diff --git a/openerp/osv/fields.py b/openerp/osv/fields.py
index 2a6d0d6e3eb..3117793dcab 100644
--- a/openerp/osv/fields.py
+++ b/openerp/osv/fields.py
@@ -181,7 +181,7 @@ class reference(_column):
 class char(_column):
     _type = 'char'
 
-    def __init__(self, string, size, **args):
+    def __init__(self, string="unknown", size=None, **args):
         _column.__init__(self, string=string, size=size, **args)
         self._symbol_set = (self._symbol_c, self._symbol_set_char)