get_option intermittently fails

I have added a theme options panel for my theme. I store the options in an array and serialize it when I store them with update_option().

When the theme options panel is submitted I call a function to validate and call update_option().
Then I call get_option(), and unserialize() to retrieve the new options. Unfortunately, get_option() sometimes fails, not returning a string.
Is there something that I can do to prevent these failures?

This is a sort of pseudo code version of my code:

function themeoptions_retrieve() {
  return unserialize(get_option('themeoptions'));  // This can fail!
}
function themeoptions_save($theme_options) {
  update_option('themeoptions', $theme_options);
}
// This function added by add_theme_page() and add_action().
function themeoptions_page() {
 if ($_POST['save']) { themeoptions_update(); }
 $theme_options = themeoptions_retrieve();
 // Display theme options page.
}
function themeoptions_update() {
  $theme_options = themeoptions_retrieve();
  // Snip - validate $_POST data and update $theme_options array.
  themeoptions_save($theme_options);
}