Invoice missing

Hi there,
I am testing with a free subscription; in Configuration:

1. Activate Invoice Feature =  YES;
2. Generate invoice for paid subscriptions only = NO; 
3. Send Invoice to subscribers = YES.
4. in the Subscription plan, Advanced Settings: free plan subscription Status = Active

However, there's no invoice;

• I see in the user profile in Subscriptions History a field Invoice Number but is blank
• in the server's Invoice folder there's no invoice;
• no invoice is sent via mail (but yes the other subscription emails)

What am I missing?
 

The main question: What's the reason of sending invoice for free subscription?

Tuan

1. In invoice settings I see I can choose whether generate the invoice for paid subscriptions only: yes or no. I assume I can select NO and generate the invoice for free subscriptions as well.
2. testing the layout of an invoice before putting the site online.

1. It is not used to generate invoice for free subscription. It is used to generate invoice for paying-subscriptions which users have not pay for it yet (subscriptions with Payment Amount, but Pending status - for example, when users use offline payment). So it is not used for free subscription

2. To test the layout, I think you should go to Membership Pro -> Subscriptions, create a subscription record manually. Then download invoice generated for that test subscription

Regards,

Tuan