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app/code/Magento/Catalog/Block/Product/ProductList/Related.php

@@ -101,6 +101,17 @@ class Related extends AbstractProduct implements IdentityInterface
 
         $this->_itemCollection->load();
 
+        if (!count($this->_itemCollection->getItems())){
+            $collection = $this->_collectionFactory->create();
+            $collection->addAttributeToSelect('*');
+            $collection->addCategoriesFilter(['in' => $product->getCategoryIds()]);
+            $collection->setVisibility($this->_catalogProductVisibility->getVisibleInCatalogIds());
+            $collection->getSelect()->order('rand()');
+            $collection->setPageSize(6);
+            $collection->addStoreFilter($product->getStoreId());
+            $this->_itemCollection =  $collection;
+        }
+
         foreach ($this->_itemCollection as $product) {
             $product->setDoNotUseCategoryId(true);
         }