diff --git a/extensions/blocks/opentable/opentable.php b/extensions/blocks/opentable/opentable.php
index 145d6b8bc8117..c12f2548d6a66 100644
--- a/extensions/blocks/opentable/opentable.php
+++ b/extensions/blocks/opentable/opentable.php
@@ -94,7 +94,7 @@ function load_assets( $attributes ) {
 	Jetpack_Gutenberg::load_assets_as_required( FEATURE_NAME );
 
 	$classes = array( sprintf( 'wp-block-jetpack-%s-theme-%s', FEATURE_NAME, get_attribute( $attributes, 'style' ) ) );
-	if ( count( $attributes['rid'] ) > 1 ) {
+	if ( array_key_exists( 'rid', $attributes ) && is_array( $attributes['rid'] ) && count( $attributes['rid'] ) > 1 ) {
 		$classes[] = 'is-multi';
 	}
 	$classes = Jetpack_Gutenberg::block_classes(
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 4d5b17fec1bbe..fc7f740b79576 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -104,6 +104,9 @@
 		<testsuite name="deprecation">
 			<file>tests/php/test_deprecation.php</file>
 		</testsuite>
+		<testsuite name="extensions">
+			<directory prefix="test" suffix=".php">tests/php/extensions</directory>
+		</testsuite>
 	</testsuites>
 	<groups>
 		<exclude>
diff --git a/tests/php/extensions/blocks/test-class.opentable.php b/tests/php/extensions/blocks/test-class.opentable.php
new file mode 100644
index 0000000000000..3d5eca9e920e2
--- /dev/null
+++ b/tests/php/extensions/blocks/test-class.opentable.php
@@ -0,0 +1,45 @@
+<?php
+/**
+ * OpenTable Block tests.
+ *
+ * @package Jetpack
+ */
+
+use Automattic\Jetpack\Extensions\OpenTable;
+require_once JETPACK__PLUGIN_DIR . '/extensions/blocks/opentable/opentable.php';
+
+/**
+ * OpenTable block tests
+ */
+class WP_Test_OpenTable extends \WP_UnitTestCase {
+
+	/**
+	 * `load_assets` with empty attributes
+	 */
+	public function test_load_assets_with_empty_attributes() {
+		$attributes = array();
+		$content    = OpenTable\load_assets( $attributes );
+
+		$this->assertTrue( is_string( $content ) );
+	}
+
+	/**
+	 * `load_assets` with `rid` attribute set to null
+	 */
+	public function test_load_assets_rid_not_valid() {
+		$attributes = array( 'rid' => null );
+		$content    = OpenTable\load_assets( $attributes );
+
+		$this->assertTrue( is_string( $content ) );
+	}
+
+	/**
+	 * `load_assets` with `rid` as array
+	 */
+	public function test_load_assets_rid_empty_array() {
+		$attributes = array( 'rid' => array() );
+		$content    = OpenTable\load_assets( $attributes );
+
+		$this->assertTrue( is_string( $content ) );
+	}
+}