"navigator.globalization.getDateNames" strange problem in wp8

when I execute these code

navigator.globalization.getDateNames(function(names) {for (var i=0; i<names.value.length; i++) { alert(i + names.value[i]);}},null,

)

[in android and ios，it will return]
0 1
1 2
2 3
...
...
...
11 12

[in wp8，it will return]
0 1
1 2
2 3
...
...
...
11 12
12 null <= this line maybe have some problem or Excess ?