diff --git a/app/controllers/registrations_controller.rb b/app/controllers/registrations_controller.rb
index 6b79b180bc2e1bf69783d6bc83d1958bd90e0b47..ec9bd7f1beb768554e5df915d0784f9717d7d57d 100644
--- a/app/controllers/registrations_controller.rb
+++ b/app/controllers/registrations_controller.rb
@@ -7,13 +7,14 @@ class RegistrationsController < Devise::RegistrationsController
     begin
       user = User.instantiate!(params[:user])
     rescue MongoMapper::DocumentNotValid => e
-      user = nil
       flash[:error] = e.message
+      redirect_to new_user_registration_path
     end
-    if user
+    if user.save
       flash[:notice] = I18n.t 'registrations.create.success'
       sign_in_and_redirect(:user, user)
     else
+      flash[:error] = user.errors.full_messages.join(', ')
       redirect_to new_user_registration_path
     end
   end
diff --git a/app/models/user.rb b/app/models/user.rb
index ec7107fc3c0d37f5868166407a80c9201b3ee237..ca9a0d8423609979266aab8b08bddf9ae10c1c7b 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -24,6 +24,7 @@ class User
 
   devise :invitable, :database_authenticatable, :registerable,
          :recoverable, :rememberable, :trackable, :validatable
+
   key :username, :unique => true
   key :serialized_private_key, String
 
@@ -36,6 +37,8 @@ class User
   key :visible_post_ids,    Array
   key :visible_person_ids,  Array
 
+  validates_presence_of :username
+
   one :person, :class_name => 'Person', :foreign_key => :owner_id
 
   many :inviters,          :in => :inviter_ids,         :class_name => 'User'
diff --git a/spec/controllers/registrations_controller_spec.rb b/spec/controllers/registrations_controller_spec.rb
index 828e6a4e4394471137d8e58dfbac669a5077e88c..da7643fc9cabc433b8f9f1967b045c59e3c610f5 100644
--- a/spec/controllers/registrations_controller_spec.rb
+++ b/spec/controllers/registrations_controller_spec.rb
@@ -35,5 +35,22 @@ describe RegistrationsController do
         response.should redirect_to root_path
       end
     end
+    context "with invalid parameters" do
+      before do
+        @valid_params["user"].delete("username")
+        @invalid_params = @valid_params
+      end
+      it "does not create a user" do
+        lambda { get :create, @invalid_params }.should_not change(User, :count)
+      end
+      it "sets the flash error" do
+        get :create, @invalid_params
+        flash[:error].should_not be_blank
+      end
+      it "goes back to the form" do
+        get :create, @invalid_params
+        response.should redirect_to new_user_registration_path
+      end
+    end
   end
 end
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index cf7c4c51a2e305fadd1fcc1404fe115287557d0c..4f8b836b2a27f010eecf57e707f1eda01f5a6bbd 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -13,6 +13,10 @@ describe User do
   let(:aspect3) { user3.aspect(:name => 'stuff') }
 
   describe "validations" do
+    it "requires a username" do
+      user = Factory.build(:user, :username => nil)
+      user.should_not be_valid
+    end
     it "downcases the username" do
       user = Factory.build(:user, :username => "ALLUPPERCASE")
       user.valid?