diff --git a/lib/devise/controllers/filters.rb b/lib/devise/controllers/filters.rb
index e5bde1d0..79ab2aae 100644
--- a/lib/devise/controllers/filters.rb
+++ b/lib/devise/controllers/filters.rb
@@ -58,7 +58,7 @@ module Devise
       #   sign_in @user           # sign_in(resource)
       #
       def sign_in(resource_or_scope, resource=nil)
-        scope    ||= find_devise_scope(resource_or_scope)
+        scope    ||= Devise::Mapping.find_scope!(resource_or_scope)
         resource ||= resource_or_scope
         warden.set_user(resource, :scope => scope)
       end
@@ -72,7 +72,7 @@ module Devise
       #   sign_out @user     # sign_out(resource)
       #
       def sign_out(resource_or_scope)
-        scope = find_devise_scope(resource_or_scope)
+        scope = Devise::Mapping.find_scope!(resource_or_scope)
         warden.user(scope) # Without loading user here, before_logout hook is not called
         warden.raw_session.inspect # Without this inspect here. The session does not clear.
         warden.logout(scope)
@@ -86,7 +86,7 @@ module Devise
       #   redirect_to stored_location_for(:user) || root_path
       #
       def stored_location_for(resource_or_scope)
-        scope = find_devise_scope(resource_or_scope)
+        scope = Devise::Mapping.find_scope!(resource_or_scope)
         session.delete(:"#{scope}.return_to")
       end
 
@@ -134,16 +134,6 @@ module Devise
         METHODS
       end
 
-      protected
-
-      def find_devise_scope(resource_or_scope) #:nodoc:
-        if resource_or_scope.is_a?(Symbol)
-          resource_or_scope
-        else
-          Devise::Mapping.find_by_class!(resource_or_scope.class).name
-        end
-      end
-
     end
   end
 end
diff --git a/lib/devise/mapping.rb b/lib/devise/mapping.rb
index 3e3d94dd..2ecd7089 100644
--- a/lib/devise/mapping.rb
+++ b/lib/devise/mapping.rb
@@ -39,11 +39,17 @@ module Devise
       Devise.mappings.values.find { |m| return m if klass <= m.to }
     end
 
-    # Find by class but raising an error in case it can't be found.
-    def self.find_by_class!(klass)
-      mapping = find_by_class(klass)
-      raise "Could not find a valid mapping for #{klass}" unless mapping
-      mapping
+    # Receives an object and find a scope for it. If a scope cannot be found,
+    # raises an error. If a symbol is given, it's considered to be the scope.
+    def self.find_scope!(duck)
+      if duck.is_a?(Symbol)
+        duck
+      else
+        klass = duck.is_a?(Class) ? duck : duck.class
+        mapping = Devise::Mapping.find_by_class(klass)
+        raise "Could not find a valid mapping for #{duck}" unless mapping
+        mapping.name
+      end
     end
 
     # Default url options which can be used as prefix.
diff --git a/lib/devise/test_helpers.rb b/lib/devise/test_helpers.rb
index 143eac9a..3ff7461c 100644
--- a/lib/devise/test_helpers.rb
+++ b/lib/devise/test_helpers.rb
@@ -64,7 +64,7 @@ module Devise
     #   sign_in @user          # sign_in(resource)
     #
     def sign_in(resource_or_scope, resource=nil)
-      scope    ||= find_devise_scope(resource_or_scope)
+      scope    ||= Devise::Mapping.find_scope!(resource_or_scope)
       resource ||= resource_or_scope
       session["warden.user.#{scope}.key"] = resource.class.serialize_into_session(resource)
     end
@@ -77,19 +77,9 @@ module Devise
     #   sign_out @user     # sign_out(resource)
     #
     def sign_out(resource_or_scope)
-      scope = find_devise_scope(resource_or_scope)
+      scope = Devise::Mapping.find_scope!(resource_or_scope)
       warden.logout(scope)
     end
 
-    protected
-
-      def find_devise_scope(resource_or_scope) #:nodoc:
-        if resource_or_scope.is_a?(Symbol)
-          resource_or_scope
-        else
-          Devise::Mapping.find_by_class!(resource_or_scope.class).name
-        end
-      end
-
   end
 end
diff --git a/test/mapping_test.rb b/test/mapping_test.rb
index 6c5946f2..ce515dbc 100644
--- a/test/mapping_test.rb
+++ b/test/mapping_test.rb
@@ -49,9 +49,15 @@ class MappingTest < ActiveSupport::TestCase
     assert_equal Devise.mappings[:user], Devise::Mapping.find_by_class(klass)
   end
 
-  test 'find mapping raises an error for invalid class' do
+  test 'find scope for a given object' do
+    assert_equal :user, Devise::Mapping.find_scope!(User)
+    assert_equal :user, Devise::Mapping.find_scope!(:user)
+    assert_equal :user, Devise::Mapping.find_scope!(User.new)
+  end
+
+  test 'find scope raises an error if cannot be found' do
     assert_raise RuntimeError do
-      Devise::Mapping.find_by_class!(String)
+      Devise::Mapping.find_scope!(String)
     end
   end