nil.go

package main

import (
	"fmt"
)

func main() {
	err := fmt.Errorf("this is an error.")

	// these two lines print the same thing when err is non-nil
	fmt.Printf("%v\n", err)
	fmt.Printf("%s\n", err.Error())

	err = nil
	fmt.Printf("%v\n", err)         // this is safe
	fmt.Printf("%s\n", err.Error()) // this is a panic
}