PageMethods calls inconsistent

Hi All,
Ran into a bit of problem when using PageMethods when calling a webmethod in the code-behind.  My markup page has a radgrid which gets populated via the objectdatasource. Pagemethods works and returns data as expected when the objectdatasource binds the radgrid with some data but not when the objectdatasource returns nothing, i.e. radgrid is empty. PageMethods call has got nothing to do with the objectdatasource, I'm simply using its output to build a list of checkboxes dynamically on the client side using Javascript.  And this is how my code snippets looks like:
Markup page:
<script type="text/javascript">
         function CallGetARInvoices(src,dest)
         {                
             ...