diff --git a/ldapcherry/__init__.py b/ldapcherry/__init__.py
index 66764c4..de56708 100644
--- a/ldapcherry/__init__.py
+++ b/ldapcherry/__init__.py
@@ -661,8 +661,16 @@ class LdapCherry(object):
                     if b not in badd:
                         badd[b] = {}
                     badd[b][backends[b]] = params['attrs'][attr]
+        added = False
         for b in badd:
-            self.backends[b].add_user(badd[b])
+            try:
+                self.backends[b].add_user(badd[b])
+                added = True
+            except UserAlreadyExists as e:
+                self._add_notification('User already exists in backend "' + b + '"')
+        if not added:
+            raise e
+
 
         key = self.attributes.get_key()
         username = params['attrs'][key]
@@ -719,7 +727,11 @@ class LdapCherry(object):
                         badd[b] = {}
                     badd[b][backends[b]] = params['attrs'][attr]
         for b in badd:
-            self.backends[b].set_attrs(username, badd[b])
+            try:
+                self.backends[b].set_attrs(username, badd[b])
+            except UserDoesntExist as e:
+                self._add_notification('User does not exist in backend "' + b + '"')
+
         return badd
 
     def _selfmodify(self, params):
@@ -1096,6 +1108,15 @@ class LdapCherry(object):
         display_names = {}
         for r in self.roles.flatten:
             display_names[r] = self.roles.flatten[r]['display_name']
+
+        if user is None:
+            cherrypy.response.status = 400
+            return self.temp['error.tmpl'].render(
+                is_admin=is_admin,
+                alert='warning',
+                message="No user requested"
+                )
+
         user_attrs = self._get_user(user)
         if user_attrs == {}:
             cherrypy.response.status = 400
diff --git a/ldapcherry/backend/backendLdap.py b/ldapcherry/backend/backendLdap.py
index c85b2ef..703dda1 100644
--- a/ldapcherry/backend/backendLdap.py
+++ b/ldapcherry/backend/backendLdap.py
@@ -342,6 +342,8 @@ class Backend(ldapcherry.backend.Backend):
         """ Set user attributes"""
         ldap_client = self._bind()
         tmp = self._get_user(self._str(username), ALL_ATTRS)
+        if tmp is None:
+            raise UserDoesntExist(username, self.backend_name)
         dn = self._str(tmp[0])
         old_attrs = tmp[1]
         for attr in attrs:
diff --git a/tests/test_LdapCherry.py b/tests/test_LdapCherry.py
index 1cd8f8a..9c5667b 100644
--- a/tests/test_LdapCherry.py
+++ b/tests/test_LdapCherry.py
@@ -288,8 +288,13 @@ class TestError(object):
     def testNoneType(self):
         app = LdapCherry()
         loadconf('./tests/cfg/ldapcherry_test.ini', app)
-        app.modify('ssmith'),
+        app.modify('ssmith')
  
+    def testNoneModify(self):
+        app = LdapCherry()
+        loadconf('./tests/cfg/ldapcherry_test.ini', app)
+        app.modify(user=None)
+
     @slow_disabled
     def testNaughtyStrings(self):
         app = LdapCherry()
@@ -317,6 +322,32 @@ class TestError(object):
         inv.add_user(addefault_user.copy())
         app._deleteuser(u'☭default_user')
 
+    @travis_disabled
+    def testAddUserOneBackend(self):
+        app = LdapCherry()
+        loadconf('./tests/cfg/ldapcherry_adldap.cfg', app)
+        inv = ldapcherry.backend.backendAD.Backend(adcfg, cherrypy.log, u'test☭', adattr, 'sAMAccountName')
+        inv.add_user(addefault_user.copy())
+        form = {'groups': {}, 'attrs': {'password1': u'password☭P455', 'password2': u'password☭P455', 'cn': u'Test ☭ Test', 'name': u'Test ☭', 'uidNumber': u'1000', 'gidNumber': u'1000', 'home': u'/home/test', 'first-name': u'Test ☭', 'email': u'test@test.fr', 'uid': u'☭default_user'}, 'roles': {'admin-lv3': u'on', 'admin-lv2': u'on', 'users': u'on'}}
+        app._adduser(form)
+        app._deleteuser(u'☭default_user')
+
+    @travis_disabled
+    def testModifyUserOneBackend(self):
+        app = LdapCherry()
+        loadconf('./tests/cfg/ldapcherry_adldap.cfg', app)
+        inv = ldapcherry.backend.backendAD.Backend(adcfg, cherrypy.log, u'test☭', adattr, 'sAMAccountName')
+	try:
+            app._deleteuser(u'☭default_user')
+        except:
+            pass
+        inv.add_user(addefault_user.copy())
+        modify_form = { 'attrs': {'first-name': u'Test42 ☭', 'uid': u'☭default_user'}, 'roles': { 'admin-lv3': u'on'}}
+        app._modify(modify_form)
+        app._deleteuser(u'☭default_user')
+
+
+
     def testLogger(self):
         app = LdapCherry()
         loadconf('./tests/cfg/ldapcherry.ini', app)