Transaction getKey is always null

[jolestar]

@Test
    public void testGetKeyFromSignature() {
        ECKey sender = new ECKey();
        BlockchainImpl blockchain = ImportLightTest.createBlockchain(GenesisLoader.loadGenesis(
                getClass().getResourceAsStream("/genesis/genesis-light.json")));
        Transaction tx = createTx(blockchain, sender, new byte[32], new byte[0], 100);
        Assert.assertNotNull(tx.getKey());
    }

tx.getKey() is always null.