diff --git a/src/saml2/pack.py b/src/saml2/pack.py
index 7ce2d5d..8f28d34 100644
--- a/src/saml2/pack.py
+++ b/src/saml2/pack.py
@@ -135,7 +135,7 @@ def http_redirect_message(message, location, relay_state="", typ="SAMLRequest",
 
         if sigalg == RSA_SHA1:
             signer = RSASigner(sha1_digest, "sha1")
-            string = "&".join([urllib.urlencode({k: args[k]}) for k in _order])
+            string = "&".join([urllib.urlencode({k: args[k]}) for k in _order if k in args])
             args["Signature"] = base64.b64encode(signer.sign(string, key))
             string = urllib.urlencode(args)
         else:
@@ -265,4 +265,4 @@ def packager(identifier):
 
 
 def factory(binding, message, location, relay_state="", typ="SAMLRequest"):
-    return PACKING[binding](message, location, relay_state, typ)
\ No newline at end of file
+    return PACKING[binding](message, location, relay_state, typ)