No contacts for the customer

No contacts for the customer

I get the following email when a payment reminder goes out to one particular customer:

Payment Reminder - No contacts for the customer John Doe Construction

Hello,

Payment reminder for invoice INV-599 dated June 01, 2018 is not yet sent to John Doe Construction, as contact details are not available for this customer. Please send the reminder manually. 

Thanks,
​
Team ZOHO


However, all contact information is in the "Contact".

Please help!