followed the documentation , i'm getting error : 'Invalid grant_type issue...

i'm trying to make a post request to get access token, but i still get this message :

{"error":"invalid_request","error_description":"Invalid grant_type parameter or parameter missing"}

the request:

$Client->post('https://api.box.com/oauth2/token', array(
'headers' => array('content-type' => 'application/x-www-form-urlencoded'), 
'body' => 'grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&client_id=rgc#####&client_secret=MH########&assertion=eyJhbGciOiJ#####.#####.####')) 

why i'm still getting this message and what migh be the main cause for this issue? thank you.