diff --git a/server/models/folders.js b/server/models/folders.js
index 4c5e9cf..5ecfca5 100644
--- a/server/models/folders.js
+++ b/server/models/folders.js
@@ -13,7 +13,7 @@ class Folders {
 
         const existingFolder = await foldersCollection.findOne({ title: title, userId: userId });
 
-        if (existingFolder) return null;
+        if (existingFolder) return new Error('Folder already exists');
 
         const newFolder = {
             userId: userId,
@@ -171,4 +171,4 @@ class Folders {
 
 }
 
-module.exports = new Folders;
\ No newline at end of file
+module.exports = new Folders;
diff --git a/server/models/users.js b/server/models/users.js
index caef0a1..3790fce 100644
--- a/server/models/users.js
+++ b/server/models/users.js
@@ -40,7 +40,7 @@ class Users {
         await userCollection.insertOne(newUser);
 
         const folderTitle = 'Dossier par Défaut'; 
-        const userId = newUser._id; 
+        const userId = newUser._id.toString(); 
         await Folders.create(folderTitle, userId);
 
         // TODO: verif if inserted properly...
@@ -118,4 +118,4 @@ class Users {
 
 }
 
-module.exports = new Users;
\ No newline at end of file
+module.exports = new Users;