input submit with Behat/MinkExtension

I have make test to add article in my project symfony but i have probleme in press input submit

this is code my feature:

Feature: ajouter

Feature: dossiertest

		@javascript
		Scenario: List 2 files in a directory
		 
		  Given I am on the homepage
		  Then I should see "hello"
		  When I go to "/ajoutermed"
		  Then I should see "ajouter"
		  And I wait 2 seconds
		  And I fill in "test_medbundle_med_section" with "test section"
		  And I fill in "test_medbundle_med_info" with "test info"
		  When I click on "ajout"
		  Then I should see "ajouter"

this is code context:

        /**
         * @When /^I click on "([^"]*)"$/
         */
        public function iClickOn($arg1)
        {
           
            $session = $this->getSession();
            $element = $this->getSession()->getPage()->findById($arg1);

        if ($element) {
            $element->click();
        } else {
            var_dump($arg1 . " could not be found");
            var_dump($element);
        }
        }

now i have this problem when i lunch my test in step " When I click on "ajout"

http://imgur.com/a/k3NJD

Start with the Behat feature scenario and the custom iClickOn step shown in the issue, then run the scenario to capture the actual failure behind the linked screenshot. Check whether the element identified as "ajout" is found and clicked; done means the submit step completes and the expected page content is displayed.