diff -r 03678acbfda3 -r 49b75287c30b server/src/app/Models/Document.php
--- a/server/src/app/Models/Document.php	Sat Mar 26 00:24:34 2016 +0100
+++ b/server/src/app/Models/Document.php	Fri Apr 08 17:50:49 2016 +0200
@@ -273,8 +273,8 @@
         //remove old,
         foreach ($this->getContributors() as $contribDef) {
             $value = null;
-            if (is_null($contribDef['url'])) {
-                if(is_null($contribDef['nameLiteral'])) {
+            if (empty($contribDef['url'])) {
+                if(empty($contribDef['nameLiteral'])) {
                     $value = new Literal($contribDef['name']);
                 } else {
                     $value = $contribDef['nameLiteral'];
@@ -289,7 +289,7 @@
         //put new
         foreach ($contributors as $newContribDef) {
             $value = null;
-            if (is_null($newContribDef['url'])) {
+            if (empty($newContribDef['url'])) {
                 $value = new Literal($newContribDef['name'], "fr", null);
             } else {
                 $value = new Resource($newContribDef['url']);