How can I make this code work? IntelliJ complains ...

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[apahl]

How can I make this code work?
IntelliJ complains that condition 'line != null' is always 'true'
and when I compile it, I get the error f.readLine() must not be null.

val f = File(fn).reader().buffered()
var line: String
line = f.readLine()
if (line != null) { ... }