diff --git a/src/Event/Core_Events.php b/src/Event/Core_Events.php
index e4fa390..9e03a85 100644
--- a/src/Event/Core_Events.php
+++ b/src/Event/Core_Events.php
@@ -14,13 +14,13 @@ class Core_Events implements Hook_Event_Config_Interface {
 	public function get_events(): array {
 
 		return [
-			[ 'Plugins Load', 'plugin', null, 'plugins_loaded', - 1, - 2 ],
+			[ 'Plugins Load', 'plugin', '', 'plugins_loaded', - 1, - 2 ],
 			[ 'Plugins Loaded Hook', 'plugin', 'plugins_loaded' ],
 			[ 'Themes Load', 'theme', 'setup_theme', 'after_setup_theme' ],
 			[ 'Core Init', 'core', 'init', 'wp_loaded' ],
 			[ 'Admin Init', 'core', 'admin_init' ],
 			[ 'Admin Menu', 'core', '_admin_menu', 'admin_menu' ],
-			[ 'Toolbar', 'core', 'admin_bar_menu', null ],
+			[ 'Toolbar', 'core', 'admin_bar_menu', '' ],
 			[ 'Main Loop', 'theme', 'loop_start', 'loop_end' ],
 			[ 'Admin Print Scripts', 'core', 'admin_enqueue_scripts', 'admin_print_scripts' ],
 		];
diff --git a/src/Record/Hook_Record_Collector.php b/src/Record/Hook_Record_Collector.php
index 564a8f4..aadd98d 100644
--- a/src/Record/Hook_Record_Collector.php
+++ b/src/Record/Hook_Record_Collector.php
@@ -66,15 +66,15 @@ class Hook_Record_Collector extends Stopwatch_Record_Collector {
 	 *
 	 * @param string      $event          Hook event name.
 	 * @param string      $category       Hook event category.
-	 * @param string|null $start          Start hook name. Pass null to ignore.
-	 * @param string|null $stop           Stop hook name (defaults to start name).
+	 * @param string      $start          Start hook name. Pass empty string to ignore.
+	 * @param string|null $stop           Stop hook name (defaults to start name). Pass empty string to ignore.
 	 * @param int         $start_priority Start hook priority (defaults to -1).
 	 * @param int         $stop_priority  Stop hook priority (defaults to max int).
 	 */
 	private function add_event(
 		string $event,
 		string $category,
-		?string $start = null,
+		string $start,
 		?string $stop = null,
 		int $start_priority = - 1,
 		int $stop_priority = PHP_INT_MAX
@@ -85,7 +85,7 @@ class Hook_Record_Collector extends Stopwatch_Record_Collector {
 
 		$collector = $this;
 
-		if ( null !== $start ) {
+		if ( '' !== $start ) {
 			add_action( $start, function ( $input = null ) use ( $collector, $event, $category ) {
 				$collector->start( $event, $category );
 
@@ -93,7 +93,7 @@ class Hook_Record_Collector extends Stopwatch_Record_Collector {
 			}, $start_priority );
 		}
 
-		if ( null !== $stop ) {
+		if ( '' !== $stop ) {
 			add_action( $stop, function ( $input = null ) use ( $collector, $event ) {
 				$collector->stop( $event );