$ERR$ doesn't detect custom error type

main.go

package main

import "fmt"

type MyError struct {
	Code int
	Message string
}

func (m MyError) Error() string {
	return fmt.Sprintf("%d: %s",m.Code,m.Message)
}

func ReturnError() MyError {
	return MyError{
		Code:1,
		Message:"kaboom",
	}
}

func main(){
	err := ReturnError()
}