diff --git a/lib/api/v3/work_packages/work_packages_api.rb b/lib/api/v3/work_packages/work_packages_api.rb
index 8635bc0c37..645c8abe5b 100644
--- a/lib/api/v3/work_packages/work_packages_api.rb
+++ b/lib/api/v3/work_packages/work_packages_api.rb
@@ -40,18 +40,28 @@ module API
 
               def write_work_package_attributes
                 if request_body
-                  payload = ::API::V3::WorkPackages::Form::WorkPackagePayloadRepresenter.create(
-                    @work_package,
-                    enforce_lock_version_validation: true)
-
                   begin
-                    payload.from_json(request_body.to_json)
+                    # we need to merge the JSON two times:
+                    # In Pass 1 the representer only has custom fields for the current WP type
+                    # After Pass 1 the correct type information is merged into the WP
+                    # In Pass 2 the representer is created with the new type info and will be able
+                    # to also parse custom fields successfully
+                    merge_json_into_work_package!(request_body.to_json)
+                    merge_json_into_work_package!(request_body.to_json)
                   rescue ::API::Errors::Form::InvalidResourceLink => e
                     fail ::API::Errors::Validation.new(e.message)
                   end
                 end
               end
 
+              # merges the given JSON representation into @work_package
+              def merge_json_into_work_package!(json)
+                payload = ::API::V3::WorkPackages::Form::WorkPackagePayloadRepresenter.create(
+                  @work_package,
+                  enforce_lock_version_validation: true)
+                payload.from_json(json)
+              end
+
               def request_body
                 env['api.request.body']
               end
diff --git a/lib/plugins/acts_as_customizable/lib/acts_as_customizable.rb b/lib/plugins/acts_as_customizable/lib/acts_as_customizable.rb
index 03a79bf23d..880d8bf15c 100644
--- a/lib/plugins/acts_as_customizable/lib/acts_as_customizable.rb
+++ b/lib/plugins/acts_as_customizable/lib/acts_as_customizable.rb
@@ -46,7 +46,6 @@ module Redmine
                                    order: "#{CustomField.table_name}.position",
                                    dependent: :delete_all,
                                    validate: false
-          before_validation { |customized| customized.custom_field_values if customized.new_record? }
           validate :validate_custom_values
           send :include, Redmine::Acts::Customizable::InstanceMethods
           # Save custom values when saving the customized object
@@ -119,7 +118,7 @@ module Redmine
         end
 
         def validate_custom_values
-          custom_values.reject(&:marked_for_destruction?).select(&:invalid?).each do |custom_value|
+          custom_field_values.reject(&:marked_for_destruction?).select(&:invalid?).each do |custom_value|
             custom_value.errors.each do |_, message|
               errors.add(custom_value.custom_field.accessor_name.to_sym, message)
             end