diff --git a/eopayment/systempayv2.py b/eopayment/systempayv2.py
index da4a470..dc1853b 100644
--- a/eopayment/systempayv2.py
+++ b/eopayment/systempayv2.py
@@ -492,10 +492,15 @@ class Payment(PaymentCommon):
             message += ' signature invalide.'
 
         test = fields[VADS_CTX_MODE] == 'TEST'
-        if VADS_EOPAYMENT_TRANS_ID in fields:
-            transaction_id = fields[VADS_EOPAYMENT_TRANS_ID]
+        vads_eopayment_trans_id = fields.get(VADS_EOPAYMENT_TRANS_ID)
+        vads_trans_date = fields.get(VADS_TRANS_DATE)
+        vads_trans_id = fields.get(VADS_TRANS_ID)
+        if vads_eopayment_trans_id:
+            transaction_id = vads_eopayment_trans_id
+        elif vads_trans_date and vads_trans_id:
+            transaction_id = vads_trans_date + '_' + vads_trans_id
         else:
-            transaction_id = '%s_%s' % (copy[VADS_TRANS_DATE], copy[VADS_TRANS_ID])
+            raise ResponseError('backend error', message)
         # the VADS_AUTH_NUMBER is the number to match payment in bank logs
         copy[self.BANK_ID] = copy.get(VADS_AUTH_NUMBER, '')
         transaction_date = None