An interface is only considered nil if it is not associated with an underlying value, even a nil value.

errors.go

package main

import "fmt"

type Error struct {
	Message string
}

func (e *Error) Error() string {
	return "an error occured"
}

func error1() error {
	var e error
	fmt.Println("error1: ", e == nil)
	return e
}

func error2() error {
	var e *Error
	fmt.Println("error2: ", e == nil)
	return e
}

func main() {
	e1 := error1()
	fmt.Println("e1:     ", e1 == nil)

	e2 := error2()
	fmt.Println("e2:     ", e2 == nil)
}