diff --git a/eopayment/ogone.py b/eopayment/ogone.py
index 136462b..fbc9e2d 100644
--- a/eopayment/ogone.py
+++ b/eopayment/ogone.py
@@ -463,7 +463,7 @@ class Payment(PaymentCommon):
         values = params.items()
         values = [(a.upper(), b) for a, b in values]
         values = sorted(values)
-        values = [u'%s=%s' % (a, b) for a, b in values if a in keep]
+        values = [u'%s=%s' % (a, b) for a, b in values if a in keep and b]
         tosign = key.join(values)
         tosign += key
         tosign = force_byte(tosign)