diff --git a/src/Plugin/search_api/backend/SearchApiSolrBackend.php b/src/Plugin/search_api/backend/SearchApiSolrBackend.php
index a7214c3f..be542351 100644
--- a/src/Plugin/search_api/backend/SearchApiSolrBackend.php
+++ b/src/Plugin/search_api/backend/SearchApiSolrBackend.php
@@ -5084,7 +5084,7 @@ class SearchApiSolrBackend extends BackendPluginBase implements SolrBackendInter
    *
    * @see getSolrConnector()
    */
-  public function __sleep() {
+  public function __sleep(): array {
     $properties = array_flip(parent::__sleep());
 
     unset($properties['solrConnector']);
diff --git a/src/SolrConnector/SolrConnectorPluginBase.php b/src/SolrConnector/SolrConnectorPluginBase.php
index b8790615..bce67004 100644
--- a/src/SolrConnector/SolrConnectorPluginBase.php
+++ b/src/SolrConnector/SolrConnectorPluginBase.php
@@ -1363,7 +1363,7 @@ abstract class SolrConnectorPluginBase extends ConfigurablePluginBase implements
   /**
    * {@inheritdoc}
    */
-  public function __sleep() {
+  public function __sleep(): array {
     // It's safe to unset the solr client completely before serialization
     // because connect() will set it up again correctly after deserialization.
     unset($this->solr);