diff --git a/src/Orchard.Web/Modules/Orchard.Users/Controllers/AccountController.cs b/src/Orchard.Web/Modules/Orchard.Users/Controllers/AccountController.cs
index c4f13ee73..e7e9effb2 100644
--- a/src/Orchard.Web/Modules/Orchard.Users/Controllers/AccountController.cs
+++ b/src/Orchard.Web/Modules/Orchard.Users/Controllers/AccountController.cs
@@ -173,12 +173,20 @@ namespace Orchard.Users.Controllers {
         }
 
         private IUser ValidateLogOn(string userNameOrEmail, string password) {
+            bool validate = true;
+
             if (String.IsNullOrEmpty(userNameOrEmail)) {
                 ModelState.AddModelError("userNameOrEmail", T("You must specify a username or e-mail."));
+                validate = false;
             }
             if (String.IsNullOrEmpty(password)) {
                 ModelState.AddModelError("password", T("You must specify a password."));
+                validate = false;
             }
+
+            if (!validate)
+                return null;
+
             var user = _membershipService.ValidateUser(userNameOrEmail, password);
             if (user == null) {
                 ModelState.AddModelError("_FORM", T("The username or e-mail or password provided is incorrect."));
@@ -188,12 +196,20 @@ namespace Orchard.Users.Controllers {
         }
 
         private bool ValidateRegistration(string userName, string email, string password, string confirmPassword) {
+            bool validate = true;
+
             if (String.IsNullOrEmpty(userName)) {
                 ModelState.AddModelError("username", T("You must specify a username."));
+                validate = false;
             }
             if (String.IsNullOrEmpty(email)) {
                 ModelState.AddModelError("email", T("You must specify an email address."));
+                validate = false;
             }
+
+            if (!validate)
+                return false;
+
             string userUnicityMessage = _userService.VerifyUserUnicity(userName, email);
             if (userUnicityMessage != null) {
                 ModelState.AddModelError("userExists", T(userUnicityMessage));
diff --git a/src/Orchard.Web/Modules/Orchard.Users/Services/MembershipService.cs b/src/Orchard.Web/Modules/Orchard.Users/Services/MembershipService.cs
index 3d44d1169..86f3dab9d 100644
--- a/src/Orchard.Web/Modules/Orchard.Users/Services/MembershipService.cs
+++ b/src/Orchard.Web/Modules/Orchard.Users/Services/MembershipService.cs
@@ -44,10 +44,9 @@ namespace Orchard.Users.Services {
         }
 
         public IUser GetUser(string username) {
-            if(username == null) {
-                throw new ArgumentNullException("username");
-            }
-            var userRecord = _userRepository.Get(x => x.NormalizedUserName == username.ToLower());
+            var lowerName = username == null ? "" : username.ToLower();
+
+            var userRecord = _userRepository.Get(x => x.NormalizedUserName == lowerName);
             if (userRecord == null) {
                 return null;
             }
@@ -55,9 +54,11 @@ namespace Orchard.Users.Services {
         }
 
         public IUser ValidateUser(string userNameOrEmail, string password) {
-            var userRecord = _userRepository.Get(x => x.NormalizedUserName == userNameOrEmail.ToLower());
+            var lowerName = userNameOrEmail == null ? "" : userNameOrEmail.ToLower();
+
+            var userRecord = _userRepository.Get(x => x.NormalizedUserName == lowerName);
             if(userRecord == null)
-                userRecord = _userRepository.Get(x => x.Email == userNameOrEmail.ToLower());
+                userRecord = _userRepository.Get(x => x.Email == lowerName);
             if (userRecord == null || ValidatePassword(userRecord, password) == false)
                 return null;