Fatal error: Call to undefined function

I am trying to call a function which is defined in functions.php. But I can’t figure out why this error is coming.

Let me tell you in detail what I am trying to accomplish: there are two header files?—?client and admin. The client header will be accessed when the user enters into a page through index.php, but admin header will be accessed after an admin successfuly logged in.

In the functions.php this is the function I have defined:

function get_my_header(){
  if(is_home()){
    get_header('client');
  }
  elseif(is_page('dashboard')){
    get_header('own');
  }
}

from index.php get_my_header() is called with no error but when I try to access the same function from the admin page I am getting this error:
Fatal error: Call to undefined function get_my_header()
The page where this error is coming has only this one line:

<?php get_my_header(); ?>

All files resides in one folder.

Please help ?