diff --git a/src/Search/Variants/SearchVariantSubsites.php b/src/Search/Variants/SearchVariantSubsites.php
index e58c9da..aea6008 100644
--- a/src/Search/Variants/SearchVariantSubsites.php
+++ b/src/Search/Variants/SearchVariantSubsites.php
@@ -2,6 +2,7 @@
 
 namespace SilverStripe\FullTextSearch\Search\Variants;
 
+use InvalidArgumentException;
 use SilverStripe\Assets\File;
 use SilverStripe\CMS\Model\SiteTree;
 use SilverStripe\FullTextSearch\Search\Indexes\SearchIndex;
@@ -48,7 +49,7 @@ class SearchVariantSubsites extends SearchVariant
 
     public function currentState()
     {
-        return (string) SubsiteState::singleton()->getSubsiteId();
+        return SubsiteState::singleton()->getSubsiteId();
     }
 
     public function reindexStates()
@@ -71,6 +72,12 @@ class SearchVariantSubsites extends SearchVariant
             return;
         }
 
+        if (is_numeric($state)) {
+            $state = (int) $state;
+        } elseif ($state !== null) {
+            throw new InvalidArgumentException("Invalid state ID. State ID should be number or null.");
+        }
+
         // Note: Setting directly to the SubsiteState because we don't want the subsite ID to be persisted
         // like Subsite::changeSubsite would do.
         SubsiteState::singleton()->setSubsiteId($state);