UIAutomation failed to tap a key

i use UIAutomation and instrument for my UI Tests and when i try to tap some letters, instrument return me an error:

Script threw an uncaught JavaScript error: target.frontMostApp().keyboard() failed to tap 'V' on line 27

A part of code:

passwordField.tap();
target.frontMostApp().keyboard().typeString("VEMO");

Has anyone some ideas about it?

Thank