getWachingKey contains private Key, and result in err while clone the wallet

[superiums]

`

	NetworkParameters params = MainNetParams.get();
	Wallet toWatch=new Wallet(params);
	System.out.println(toWatch.toString());
	DeterministicKey watchingKey = toWatch.getWatchingKey();
	System.out.format("key priv: %s\n", watchingKey.getPrivateKeyAsHex());

	watchingKey.dropPrivateBytes().dropParent();

	System.out.format("after drop.\nkey data: %s\n", watchingKey.toString());
	System.out.format("key priv: %s\n", watchingKey.getPrivateKeyAsHex());
	
	Wallet watchingWallet = Wallet.fromWatchingKey(params, watchingKey);

`

Exception in thread "main" java.lang.IllegalArgumentException: Private subtrees not currently supported: if you got this key from DKC.getWatchingKey() then use .dropPrivate().dropParent() on it first. at com.google.common.base.Preconditions.checkArgument(Preconditions.java:122) at org.bitcoinj.wallet.DeterministicKeyChain.<init>(DeterministicKeyChain.java:318) at org.bitcoinj.wallet.DeterministicKeyChain.watch(DeterministicKeyChain.java:351) at org.bitcoinj.wallet.KeyChainGroup.<init>(KeyChainGroup.java:92) at org.bitcoinj.wallet.Wallet.fromWatchingKey(Wallet.java:279) at shen.Test.main(Test.java:32)

[superiums]

getPrivateKeyAsHex() always returns a privateKey. i think the watchingkey should not contain privatekey,right?
and .dropPrivateBytes().dropParent() not work.

I'm using the source for BCH , cloned by git.