MockK System.getenv only

Language: Kotlin

The goal is to test a method that calls System.getenv internally, I'd like to make this method produce a predefined result

Tried:

mockkStatic(System::class)

    every {
        System.getenv(any())
    } answers {
        when {
            firstArg() as String == "clientId" -> "aaa"
            firstArg() as String == "clientSecret" -> "bbb"
            else -> throw IllegalArgumentException("something went wrong")
        }
    }

    mockkStatic(System::class)

This appears to have sent the JVM into a bit of tipsy:

Exception in thread "main" java.lang.StackOverflowError
Exception: java.lang.StackOverflowError thrown from the UncaughtExceptionHandler in thread "main"

How does one test a method like this? (as you can imagine, a lot of code uses environment variables)