snappy-go无法解压缩字符串？

Code:

    stringReader := strings.NewReader(pbMsg.GetBody().GetReqData().GetContent())
    conBytes := make([]byte,1024)
    snappyReader := snappy.NewReader(stringReader)
    rNum, _ := snappyReader.Read(conBytes)
    fmt.Println("rNum", rNum)

I use the snappy-go in a protobuf project.The pb-string was compressed by snappy.

Now I want to to de-compress the string and get the pb-string, but when I use the *string.Reader as a parameter the rNum is 0. Where's the error?

tips: pbMsg.GetBody().GetReqData().GetContent() returns a string (compressed by snappy).

thank you very much!!