diff --git a/openerp/osv/orm.py b/openerp/osv/orm.py
index 2341b92e169..6e91ba46721 100644
--- a/openerp/osv/orm.py
+++ b/openerp/osv/orm.py
@@ -3125,7 +3125,7 @@ class BaseModel(object):
                                 ('numeric', 'float', get_pg_type(f)[1], '::'+get_pg_type(f)[1]),
                                 ('float8', 'float', get_pg_type(f)[1], '::'+get_pg_type(f)[1]),
                             ]
-                            if f_pg_type == 'varchar' and f._type == 'char' and ((f.size is None and f_pg_size) or f_pg_size < f.size):
+                            if f_pg_type == 'varchar' and f._type == 'char' and f_pg_size and (f.size is None or f_pg_size < f.size):
                                 cr.execute('ALTER TABLE "%s" RENAME COLUMN "%s" TO temp_change_size' % (self._table, k))
                                 cr.execute('ALTER TABLE "%s" ADD COLUMN "%s" %s' % (self._table, k, pg_varchar(f.size)))
                                 cr.execute('UPDATE "%s" SET "%s"=temp_change_size::%s' % (self._table, k, pg_varchar(f.size)))