Fatal error: Call to a member function cacheObject() on a non-object

I just logged into to check my site and make some more edits and got this error message:
Fatal error: Call to a member function cacheObject() on a non-object
then it tells the exact line I need to look at in my subject.php file. It specifies the second line below: if ($this->_objects[$object][$object_id]->cacheObject() === true){

I’m not sure what to do!? I’m new. Can someone please help!

//set update cache flag to true if object can be cached
if ($this->_objects[$object][$object_id]->cacheObject() === true){
$this->_updateCache = true;
}