diff --git a/includes/classes/Indexable/Post/Post.php b/includes/classes/Indexable/Post/Post.php
index 5385e07e8..7a4ec1d7c 100644
--- a/includes/classes/Indexable/Post/Post.php
+++ b/includes/classes/Indexable/Post/Post.php
@@ -597,7 +597,7 @@ function_exists( 'WC' ) ? 'woocommerce_thumbnail' : 'thumbnail',
 	/**
 	 * Prepare date terms to send to ES.
 	 *
-	 * @param string $date_to_prepare Post date
+	 * @param null|string $date_to_prepare Post date
 	 * @since 0.1.4
 	 * @return array
 	 */
@@ -618,7 +618,7 @@ public function prepare_date_terms( $date_to_prepare ) {
 
 		// Combine all the date term formats and perform one single call to date_i18n() for performance.
 		$date_format    = implode( '||', array_values( $terms_to_prepare ) );
-		$combined_dates = explode( '||', date_i18n( $date_format, strtotime( $date_to_prepare ) ) );
+		$combined_dates = explode( '||', date_i18n( $date_format, strtotime( (string) $date_to_prepare ) ) );
 
 		// Then split up the results for individual indexing.
 		$date_terms = [];