diff --git a/lib/SimpleSAML/Bindings/SAML20/HTTPPost.php b/lib/SimpleSAML/Bindings/SAML20/HTTPPost.php
index 4c934ae2f67241576385d681200a5bb839a8da50..b0a840aaa5004c775325bd755aa2acaf839a44a5 100644
--- a/lib/SimpleSAML/Bindings/SAML20/HTTPPost.php
+++ b/lib/SimpleSAML/Bindings/SAML20/HTTPPost.php
@@ -192,10 +192,12 @@ class SimpleSAML_Bindings_SAML20_HTTPPost {
 		
 		} else {
 
-			SimpleSAML_Utilities::postRedirect($destination, array(
-				'RelayState' => $relayState,
-				'SAMLResponse' => base64_encode($response),
-			));
+			$data = array('SAMLResponse' => base64_encode($response));
+			if ($relayState !== NULL) {
+				$data['RelayState'] = $relayState;
+			}
+
+			SimpleSAML_Utilities::postRedirect($destination, $data);
 		}