Correct stamp field name. It failed on Dalvik (Android)

This patch prevents a really annoying issue on Android : 
https://code.google.com/p/android/issues/detail?can=2&start=0&num=100&q=&colspec=ID%20Type%20Status%20Owner%20Summary%20Stars&groupby=&sort=&id=64514

Please accept the PR, it should not impact other VMs.