Decrypting AES/ECB in Windows Store App

I'm trying to decrypt some data I'm receiving through HttpClient.

The data is encrypted using AES/ECB with a single synchronous key and padded using PKCS#5.

Currently i'm trying -

function decryptBlob(blob) {
    var Crypto = Windows.Security.Cryptography;
    var key = "M02cnQ52Ji97wvT7";

    var SAP = Crypto.Core.SymmetricKeyAlgorithmProvider.openAlgorithm("AES_ECB_PKCS7");
    var bufferedKey = Crypto.CryptographicBuffer.convertStringToBinary(key, Crypto.BinaryStringEncoding.Utf8)
    var AES = SAP.createSymmetricKey(bufferedKey);
    var data = Crypto.CryptographicBuffer.decodeFromBase64String(blob);

    var iBuffer = Crypto.Core.CryptographicEngine.decrypt(AES, data, null);
}

But this gives me the error :

0x80090005 - JavaScript runtime error: Bad Data.

In the function call:

var data = Crypto.CryptographicBuffer.decodeFromBase64String(blob);

Any ideas of what errors i'm making or how i'm going about this wrong?