Guard statement in conditional statement

Depending on the value of the units parameter, I would like to unwrap the Fahrenheit or Celsius temperature using guard. However, I get an error about

Use of unresolved identifier 'temp'

from the following example code

let units = 0

if units == 0 {
  guard let temp = currentDict["temp_f"] as? String else { return nil }
} else {
  guard let temp = currentDict["temp_c"] as? String else { return nil }
}

Why does guard not work in this example?