Using the API to add a limitation based on subtree to a role fails, with InvalidArgumentException

steps to reproduce:

• execute the following command

  $repository = $this->getContainer()->get( 'ezpublish.api.repository' );
  $repository->setCurrentUser( $repository->getUserService()->loadUser( 14 ) );
  $roleService = $repository->getRoleService();
  
  $roleCreate = $roleService->newRoleCreateStruct( "testrole" );
  $role = $roleService->createRole( $roleCreate );
  $roleService->newPolicyCreateStruct( "content", "read" );
  
  $policyStruct->addLimitation( new SubtreeLimitation, array( "limitationValues" => array( "/1/2/") ) );
  $roleService->addPolicy( $role, $policyStruct );
  

• the command will fail with:

  [eZ\Publish\Core\Base\Exceptions\InvalidArgumentException]                                                 
    Argument 'limitations' is invalid: Some validations did not validate:                                      
     array (                                                                                                   
      'Subtree' =>                                                                                             
      array (                                                                                                  
        0 =>                                                                                                   
        eZ\Publish\Core\FieldType\ValidationError::__set_state(array(                                          
           'singular' => 'limitationValues[%key%] => \'%value%\' does not exist in the backend :: /1/2/ :: ',  
           'plural' => NULL,                                                                                   
           'values' =>                                                                                         
          array (                                                                                              
            'value' => '/1/2/',                                                                                
            'key' => 0,                                                                                        
          ),                                                                                                   
        )),                                                                                                    
      ),                                                                                                       
    )