There is a small bug in Magento 2.0.4 if there is no meta description of a product, Magento by default takes the product description but does not strip it so that all the html tags are out. Find the file : vendor\magento\module-catalog\Block\Product\View.php and the code below


$currentCategory = $this->_coreRegistry->registry('current_category');
137if ($keyword) {
138    $this->pageConfig->setKeywords($keyword);
139} elseif ($currentCategory) {
140    $this->pageConfig->setKeywords($product->getName());
141}
142$description = $product->getMetaDescription();
143if ($description) {
144    $this->pageConfig->setDescription($description);
145} else {
146    $this->pageConfig->setDescription($this->string->substr($product->getDescription(), 0, 255));
147}
148if ($this->_productHelper->canUseCanonicalTag()) {
149    $this->pageConfig->addRemotePageAsset(
150        $product->getUrlModel()->getUrl($product, ['_ignore_category' => true]),
151        'canonical',
152        ['attributes' => ['rel' => 'canonical']]
153    );
154}
155

156$pageMainTitle = $this->getLayout()->getBlock('page.main.title');

Change the indicated line in blue to : $this->pageConfig->setDescription($this->string->substr(strip_tags($product->getDescription()), 0, 255));