unmarshalling map into *string has odd behaviour

The following program has this output:

    "foo: ''\n" -> map[string]string{"foo":""}
    "foo: ''\n" -> map[string]*string{}
    "foo: null\n" -> map[string]string{"foo":"null"}
    "foo: null\n" -> map[string]*string{"foo":(*string)(nil)}

Line 1 is as expected.
Line 2 is unexpected - why is the foo attribute disappearing?
Line 3 is unexpected - should an unquoted null really turn into the empty string?
Line 4 is as expected.

 package main
 import (
  "fmt"
  "launchpad.net/goyaml"
 )

 func main() {
  parse("foo: ''\n", make(map[string]string))
  parse("foo: ''\n", make(map[string]*string))
  parse("foo: null\n", make(map[string]string))
  parse("foo: null\n", make(map[string]*string))
 }

 func parse(s string, x interface{}) {
  fmt.Printf("%q ", s)
  err := goyaml.Unmarshal([]byte(s), x)
  if err != nil {
   fmt.Printf("-> error %v\n", err)
   return
  }
  fmt.Printf("-> %#v\n", x)
 }