Error in decrypting account in wallet

[StrangerZiokan]

When there is an existing wallet and the wallet file ".wlt " is available, then there is something missing that prevents decryption of the private keys using password in the wallet.

var common = nem.model.objects.create("common")("Password of NEM wallet","");
var walletAccount = wallet.accounts[0];
nem.crypto.helpers.passwordToPrivatekey(common , walletAccount , wallet.algo);
console.log(common);

Gives error as -
C:\node_modules\crypto-js\core.js:386
var hexStrLength = hexStr.length;

TypeError: Cannot read property 'length' of undefined