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Chapter 34 - does not conform to expected type 'Decoder'

iOS Apprentice Early Access v8.0.0

I’m getting: Argument type ‘CLPlacemark’ does not conform to expected type ‘Decoder’

in the updateLabels function, this code fragment:
if let placemark = placemark {
addressLabel.text = String(from: placemark)
(error is on second line)

When I search for this error, it seems to tell me that I need to make a struct ‘Decodable’, but we are not using a struct here.