Password Vault Failure

Question

I have a Javascript App in the Windows Store. From the feedback I got in the Store, my app has some problem with authentication. I narrowed it down and found out that the problem could result from a PasswordVault failure when trying to write to the vault. 

Here is my code and I wonder what kind of exception could be thrown by PasswordVault API and why it happens?

function setValueInVault(key, value) {
        try {
            var vault = new Windows.Security.Credentials.PasswordVault();
            var cred = new Windows.Security.Credentials.PasswordCredential(
                PASSWORD_VAULT_RESOURCE_NAME, key, value);
            vault.add(cred);
        }
        catch (e) {
            
        }
    }

Thanks.

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Louis