diff --git a/op/op.AddDocument.php b/op/op.AddDocument.php
index f8fac3c1c..2e4890193 100644
--- a/op/op.AddDocument.php
+++ b/op/op.AddDocument.php
@@ -80,7 +80,7 @@ $version_comment = !empty($_POST["version_comment"]) ? trim($_POST["version_comm
 if($version_comment == "" && isset($_POST["use_comment"]))
 	$version_comment = $comment;
 
-$keywords = trim($_POST["keywords"]);
+$keywords = isset($_POST["keywords"]) ? trim($_POST["keywords"]) : '';
 $categories = isset($_POST["categories"]) ? $_POST["categories"] : null;
 $cats = array();
 if($categories) {