Throw error if command-line option to sls deploy is missing

I have created a Serverless project to allow other users to spin us an AWS lambda based on some parameters they pass as command-line args when running sls deploy --brand testbrand. One of them is used in the service name as so throws:

Serverless Error ---------------------------------------
  Trying to populate non string value into a string for variable ${opt:brand}. Please make sure the value of the property is a string.

if it is omitted. But the others are used elsewhere and only throw a warning. If there a way to make certain command-line args compulsory and throw an error if they are ommitted