How do I tell Kotlin that a method can return null...

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[dstarcev]

How do I tell Kotlin that a method can return null? This code throws java.lang.IllegalStateException: code must not be null if getHeader returns null

val code = request.getHeader("X-Confirmation-Code")
        if (code != null && codeAuthentication.isValidCode(phone, code)) {
            codeAuthentication.deleteCode(phone)
            return true
        }