diff --git a/lib/doorkeeper/models/access_token_mixin.rb b/lib/doorkeeper/models/access_token_mixin.rb
index 7ab79a786..bafb2ba46 100644
--- a/lib/doorkeeper/models/access_token_mixin.rb
+++ b/lib/doorkeeper/models/access_token_mixin.rb
@@ -10,9 +10,16 @@ module AccessTokenMixin
     include ActiveModel::MassAssignmentSecurity if defined?(::ProtectedAttributes)
 
     included do
-      belongs_to :application,
-                 class_name: 'Doorkeeper::Application',
-                 inverse_of: :access_tokens
+      belongs_to_options = {
+        class_name: 'Doorkeeper::Application',
+        inverse_of: :access_tokens
+      }
+
+      if defined?(ActiveRecord) && defined?(ActiveRecord::Base) && self < ActiveRecord::Base
+        belongs_to_options.merge! :optional => true if ActiveRecord::VERSION::MAJOR >= 5
+      end
+
+      belongs_to :application, belongs_to_options
 
       validates :token, presence: true, uniqueness: true
       validates :refresh_token, uniqueness: true, if: :use_refresh_token?
diff --git a/spec/models/doorkeeper/access_token_spec.rb b/spec/models/doorkeeper/access_token_spec.rb
index ff0e9ba3c..e28796726 100644
--- a/spec/models/doorkeeper/access_token_spec.rb
+++ b/spec/models/doorkeeper/access_token_spec.rb
@@ -143,6 +143,12 @@ module NoGenerate
         subject.resource_owner_id = nil
         expect(subject).to be_valid
       end
+
+      it 'is valid without application_id' do
+        # For resource owner credentials flow
+        subject.application_id = nil
+        expect(subject).to be_valid
+      end
     end
 
     describe '#same_credential?' do