CreateAccount stopped working on GO Client SDK on latest version

Question

I have the following code that used to work:

    client := n.getClient()

    // get the account details for master account
    masterKey := keypair.Master(n.passphrase)
    ar := horizonclient.AccountRequest{AccountID: masterKey.Address()}
    masterAccount, err := client.AccountDetail(ar)
    if err != nil {
        // error getting master account details (it usually happens when horizon is not working)
        return horizon.Transaction{}, err
    }

    // create the operation
    op := txnbuild.CreateAccount{
        Destination: "GAXFXGYRBTINYDICJMLJSPMY5BCRGAW2MGGQCNP6JB7NK5HFLB7KIZJT",
        Amount:      "10",
    }

    // build the transaction
    txn, err := txnbuild.NewTransaction(
        txnbuild.TransactionParams{
            SourceAccount:        &masterAccount,
            IncrementSequenceNum: true,
            Operations:           []txnbuild.Operation{&op},
            BaseFee:              txnbuild.MinBaseFee,
            Timebounds:           txnbuild.NewTimeout(300),
        },
    )

    if err != nil {
        // Error building the transaction
        return horizon.Transaction{}, err
    }

    // sign the transaction
    signedTxn, err := txn.Sign(n.passphrase, masterKey.(*keypair.Full))
    if err != nil {
        // error signing the transaction
        return horizon.Transaction{} ,err
    }

    // submit transaction
    return client.SubmitTransaction(signedTxn)

which is pretty much the same as the example given in the GO SDK on txnbuild.

Unfortunately, since I upgraded to the latest version of Stellar, the code is not working any longer. I am getting 400 response with txn_not_supported on extra information on the error.

Any help would be really appreciated.