diff --git a/plugins/optimization-detective/class-od-url-metric-group-collection.php b/plugins/optimization-detective/class-od-url-metric-group-collection.php
index 2c1ced1e77..ce528ec77c 100644
--- a/plugins/optimization-detective/class-od-url-metric-group-collection.php
+++ b/plugins/optimization-detective/class-od-url-metric-group-collection.php
@@ -36,10 +36,10 @@ final class OD_URL_Metric_Group_Collection implements Countable, IteratorAggrega
 	private $groups;
 
 	/**
-	 * The current Etag.
+	 * The current ETag.
 	 *
 	 * @since n.e.x.t
-	 * @var string
+	 * @var non-empty-string
 	 */
 	private $current_etag;
 
@@ -101,11 +101,11 @@ final class OD_URL_Metric_Group_Collection implements Countable, IteratorAggrega
 	 *
 	 * @throws InvalidArgumentException When an invalid argument is supplied.
 	 *
-	 * @param OD_URL_Metric[] $url_metrics   URL Metrics.
-	 * @param string          $current_etag  The current ETag.
-	 * @param int[]           $breakpoints   Breakpoints in max widths.
-	 * @param int             $sample_size   Sample size for the maximum number of viewports in a group between breakpoints.
-	 * @param int             $freshness_ttl Freshness age (TTL) for a given URL Metric.
+	 * @param OD_URL_Metric[]  $url_metrics   URL Metrics.
+	 * @param non-empty-string $current_etag  The current ETag.
+	 * @param int[]            $breakpoints   Breakpoints in max widths.
+	 * @param int              $sample_size   Sample size for the maximum number of viewports in a group between breakpoints.
+	 * @param int              $freshness_ttl Freshness age (TTL) for a given URL Metric.
 	 */
 	public function __construct( array $url_metrics, string $current_etag, array $breakpoints, int $sample_size, int $freshness_ttl ) {
 		$this->current_etag = $current_etag;
@@ -176,7 +176,7 @@ public function __construct( array $url_metrics, string $current_etag, array $br
 	 *
 	 * @since n.e.x.t
 	 *
-	 * @return string Current ETag.
+	 * @return non-empty-string Current ETag.
 	 */
 	public function get_current_etag(): string {
 		return $this->current_etag;
diff --git a/plugins/optimization-detective/class-od-url-metric.php b/plugins/optimization-detective/class-od-url-metric.php
index ad621a740c..68a5555f4a 100644
--- a/plugins/optimization-detective/class-od-url-metric.php
+++ b/plugins/optimization-detective/class-od-url-metric.php
@@ -38,7 +38,7 @@
  *                            }
  * @phpstan-type Data         array{
  *                                uuid: non-empty-string,
- *                                etag?: string,
+ *                                etag?: non-empty-string,
  *                                url: non-empty-string,
  *                                timestamp: float,
  *                                viewport: ViewportRect,
@@ -433,7 +433,7 @@ public function get_uuid(): string {
 	 *
 	 * @since n.e.x.t
 	 *
-	 * @return string|null ETag.
+	 * @return non-empty-string|null ETag.
 	 */
 	public function get_etag(): ?string {
 		// Since the ETag is optional for now, return null for old URL Metrics that do not have one.
diff --git a/plugins/optimization-detective/storage/data.php b/plugins/optimization-detective/storage/data.php
index 6d75542777..2c75f1c7d2 100644
--- a/plugins/optimization-detective/storage/data.php
+++ b/plugins/optimization-detective/storage/data.php
@@ -151,7 +151,7 @@ function od_get_url_metrics_slug( array $query_vars ): string {
  * @access private
  *
  * @param OD_Tag_Visitor_Registry $tag_visitor_registry Tag visitor registry.
- * @return string Current ETag.
+ * @return non-empty-string Current ETag.
  */
 function od_compute_current_etag( OD_Tag_Visitor_Registry $tag_visitor_registry ): string {
 	$data = array(