diff --git a/lib/SimpleSAML/Auth/ProcessingChain.php b/lib/SimpleSAML/Auth/ProcessingChain.php
index 8a718524c9b34a36af09a410d7434a4d188a6d3d..228274220020dbae24d63351ecd0875551cad702 100644
--- a/lib/SimpleSAML/Auth/ProcessingChain.php
+++ b/lib/SimpleSAML/Auth/ProcessingChain.php
@@ -344,6 +344,7 @@ class SimpleSAML_Auth_ProcessingChain {
 
 		if (count($uid) > 1) {
 			SimpleSAML\Logger::warning('Multiple attribute values for user id attribute [' . $attributeName . '].');
+			return;
 		}
 
 		$uid = $uid[0];