Index: ApiQueryUserContributions.php
===================================================================
--- ApiQueryUserContributions.php	(revision 59951)
+++ ApiQueryUserContributions.php	(working copy)
@@ -74,6 +74,8 @@
 			$this->usernames = array();
 			if(!is_array($this->params['user']))
 				$this->params['user'] = array($this->params['user']);
+			if( !count( $this->params['user'] ) )
+				$this->dieUsage( 'User parameter may not be empty', 'param_user' );
 			foreach($this->params['user'] as $u)
 				$this->prepareUsername($u);
 			$this->prefixMode = false;