tabIndex in flash?

I am trying to give tabIndex in flash.
The script is:
this.f_name.tabIndex = 1;
this.m_name.tabIndex = 2;
this.l_name.tabIndex = 3;
this.addCart.tabIndex = 4;

f_name, m_name, and l_name these are the name of text field and “addCart” is name of the button. Its successfully working on the first three text field after that its not comming on the button named “addCart”.

Please help me out