(_ textField: UITextField) resulting in "Extraneous _ parameter..."

Page 76 has us create a celsiusLabel and fahrenheitFieldEditingChanged function in the new ConversionViewController.swift file.

Is this yellow dot [message not included] (“Extraneous _ parameter: ‘textField’ has no keyword argument name” normal?

The book doesn’t mention that it will pop up and Xcode suggests to delete the underscore so that the function input line is (textField: UITextField).

Thanks for any help.