How do I test code that requires an Environment Variable?

I have some code that requires an Environment Variable to run correctly. But when I run my unit tests, it bombs out once it reaches that point unless I specifically export the variable in the terminal. I am using Scala and sbt. My code does something like this:

class something() {

  val envVar = sys.env("ENVIRONMENT_VARIABLE")

  println(envVar)

}

How can I mock this in my unit tests so that whenever sys.env("ENVIRONMENT_VARIABLE") is called, it returns a string or something like that?