updating the price for cart item give error

Question

i am updating the cart after user login for that

Error

Fatal error: Call to a member function getStoreId() on null in /opt/lampp/htdocs/instantsearch/app/code/core/Mage/Sales/Model/Quote/Item/Abstract.php on line 81

Code

foreach ($items->getAllItems() as $item) {
  $product_id = $item->getProductId();
  $customprice = 50; 
  if ($customprice > 0) 
      {  
       $item= Mage::getModel('sales/quote_item')->load($item->getId()); 
       $item->setCustomPrice($customprice);
       $item->setOriginalCustomPrice($customprice);
       $item->save(); 
      }   
    } 

Answer 1

after searching a lot i figure setProduct() was the issue

foreach ($items->getAllItems() as $item) {
      $product_id = $item->getProductId();
      $product=Mage::getModel('catalog/product')->load($product_id);
      $customprice = 50; 
      if ($customprice > 0) 
          {  
           $item= Mage::getModel('sales/quote_item')->load($item->getId()); 
           $item->setCustomPrice($customprice);
           $item->setOriginalCustomPrice($customprice);
           $item->setProduct($product);  
           $item->save(); 
          }   
        }