TypeError when encrypting data using the WebAPI window.crypto.subtle.encrypt()

I ran into a problem when I tried to use the Crypto API on firefox. (doc) I get a TypeError when I try to encrypt a clear text using this function :

window.crypto.subtle.encrypt(algo_enc,key,padded_clear_txt);

(doc)

This is how I defined my parameters:

algo_enc:

var iv = new Int32Array(4) ;//4-32 bit integers (128 bits)    
window.crypto.getRandomValues(iv); //defining the IV
var algo_enc = {"name": "AES-CBC", iv}

key:

var alg_key = {"name":"AES-CBC","length":128};   
var key = window.crypto.subtle.generateKey(alg_key,false,["encrypt","decrypt"]);

padded-clear-txt is a 256 bits (2*128) message that I want to encrypt.

This is the error I get when I execute the encrypt function :

Argument 2 of SubtleCrypto.encrypt does not implement interface CryptoKey.

The generation of the key went well, and it's a CryptoKey object, but I still get this error. So maybe this is a bug I should report...

I anyone knows I'd be grateful !