生成go文件时出现未定义的错误

I have a simple go file like below

package main

import (
    "flag"
)

var port = flag.Int("port", 23456, "port to listen.")

func main() {
    flag.Parse()
}

It was no problem when I built it yesterday, but after upgrade to go1.2.1, I get below error.

d:\dev\golang>go build main.go
# flag
C:\Go\src\pkg\flag\flag.go:87: undefined: strconv.ParseBool

Tried to uninstall 1.2.1 and re-install 1.2, same result.

Can anybody tell me why? I really appreciate any help you can provide.

go version: go1.2.windows-amd64, go1.2.1.windows-amd64
windows 7 64 bit

Okay, finally I found the solution.

After I renamed C:\Go\pkg\windows_amd64 to windows_amd64\windows_amd64_111, system automatically recreated a C:\Go\pkg\windows_amd64 folder for me, and then, everything goes OK.

Still don't know why...