main.go：9：使用没有选择器的str包

I have the following in the intepreter of Tour of Go:

package main

import (
    "golang.org/x/tour/wc"
    str "strings"
)

func WordCount(s string) map[string]int {
    results := make(map[str]int)
    words := str.Fields(s)
    return map[string]int{"x": 1}
}

//func main() {
//  wc.Test(WordCount)
//}

This is based on https://tour.golang.org/moretypes/23

My error is

tmp/sandbox169629521/main.go:9: use of package str without selector

Trying

results := make(map[str.string]int)

now fails with

tmp/sandbox424441423/main.go:9: cannot refer to unexported name strings.string
tmp/sandbox424441423/main.go:9: undefined: strings.string

"string" is a builtin. You don't need to do strings.string:

Docs: https://golang.org/pkg/builtin/#string