White Screen: PHP Fatal error: Cannot unset string offsets

Well, I am getting a white screen with this error:

PHP Fatal error: Cannot unset string offsets

This is reported at line 87, which includes this code:

  if (!$form_values['blocktheme']) {
    unset($blocktheme[$var_name]);
  } else {    
    $blocktheme[$var_name] = $form_values['blocktheme'];
  }

This was not a problem a few weeks back. I've installed several modules since then, including imagefield, simplenews template, ad, wysiwyg, imageapi, and jquery_update.

I don't understand the nature of the error. Are others running into the problem?