Domain Mapping - Provising of new sites using whmcs and mu prov

I am trying to offer two types of products via the whmcs and mu prov plugins. Everything is working well, a client is able to purchase a site when it has a domain name. However when I have a product that does not requires a domain name the provisioning receives an error stating that either

Error: : Is not a valid domain name, alphanumeric and "-" only: (Occurs when the field default domain name is left blank)

Error: Mapped domain already exists for :

I recieve this Error: Mapped domain already exists for : when I have a default blog domain inserted into the product - according to the instructions, it was my understanding that if i entered a domain name here, the name would have an auto generated number appended to the name so that I would receive this error.