golang cannot use nil as type string in return argument

 Forgive my noobness, but why can't I return nil for string return?

 
I get "cannot use nil as type string in return argument" with the following:
 
func nextLine(src *bufio.Reader) (string, os.Error) {
line, isPrefix, err := src.ReadLine()
// discard the rest of long lines
for isPrefix {
_, isPrefix, err = src.ReadLine()
}
if line == nil {
return nil, nil
}
if err != nil {
return nil, err
}
line_s := string(line)
return line_s, nil
}
 
答案:use "" for string nils.