WHMCS - Domains must begin with a letter or number - SOLVED

If you're checking for the availability of a domain on the client side of WHMCS and encounter the following error:

Domains must begin with a letter or number

If you have only set the domain pricing for one currency, and you have two or more currencies added in the system then you will receive the above error. To solve this I set the pricing for both currencies in domain pricing, or removed the unused currency and everything functioned as per normal.