Error Code: -5, Invalid private key encoding for Litecoin Private Key

I am using litecore-lib in nodejs.

Code:

Code:
var pkey = new litecore.PrivateKey();
var address = pkey.toAddress();

spawn('litecoin-cli',['importprivkey',pkey]);

But it gives me Error Message:

error code: -5
error message:
Invalid private key encoding

Please Help!!?

Reference: https://github.com/litecoin-project/litecore-lib/blob/master/docs/examples.md#generate-a-random-address