=== modified file 'l10n_be/wizard/l10n_be_vat_intra.py'
--- l10n_be/wizard/l10n_be_vat_intra.py	2013-04-29 18:29:22 +0000
+++ l10n_be/wizard/l10n_be_vat_intra.py	2013-06-19 08:47:37 +0000
@@ -80,6 +80,14 @@
         'tax_code_id': _get_tax_code,
     }
 
+    def _isdigit(self, cr, uid, ids, context=None):        
+        for wiz in self.browse(cr, uid, ids, context=context):
+            return wiz.period_code.isdigit()
+
+    _constraints = [
+        (_isdigit, 'Period can only contain digits', ['period_code']),
+    ]
+
     def _get_datas(self, cr, uid, ids, context=None):
         """Collects require data for vat intra xml
         :param ids: id of wizard.