diff --git a/src/wp-includes/nav-menu-template.php b/src/wp-includes/nav-menu-template.php
index e1a58b06689c7..135d35e13024f 100644
--- a/src/wp-includes/nav-menu-template.php
+++ b/src/wp-includes/nav-menu-template.php
@@ -638,15 +638,38 @@ function _nav_menu_item_id_use_once( $id, $item ) {
 /**
  * Remove the `menu-item-has-children` class from bottom level menu items.
  *
+ * This runs on the {@see 'nav_menu_css_class'} filter. The $args and $depth
+ * parameters were added after the filter was originally introduced in
+ * WordPress 3.0.0 so this needs to allow for cases in which the filter is
+ * called without them.
+ *
+ * @see https://core.trac.wordpress.org/ticket/56926.
+ *
  * @since 6.1.2
  *
- * @param string[] $classes   Array of the CSS classes that are applied to the menu item's `<li>` element.
- * @param WP_Post  $menu_item The current menu item object.
- * @param stdClass $args      An object of wp_nav_menu() arguments.
- * @param int      $depth     Depth of menu item.
+ * @param string[]       $classes   Array of the CSS classes that are applied to the menu item's `<li>` element.
+ * @param WP_Post        $menu_item The current menu item object.
+ * @param stdClass|false $args      An object of wp_nav_menu() arguments. Default false ($args unspecified when filter is called).
+ * @param int|false      $depth     Depth of menu item. Default false ($depth unspecified when filter is called).
  * @return string[] Modified nav menu classes.
  */
-function wp_nav_menu_remove_menu_item_has_children_class( $classes, $menu_item, $args, $depth ) {
+function wp_nav_menu_remove_menu_item_has_children_class( $classes, $menu_item, $args = false, $depth = false ) {
+	/*
+	 * Account for the filter being called without the $args or $depth parameters.
+	 *
+	 * This occurs when a theme uses a custom walker calling the `nav_menu_css_class`
+	 * filter using the legacy formats prior to the introduction of the $args and
+	 * $depth parameters.
+	 *
+	 * As both of these parameters are required for this function to determine
+	 * both the current and maximum depth of the menu tree, the function does not
+	 * attempt to remove the `menu-item-has-children` class if these parameters
+	 * are not set.
+	 */
+	if ( false === $depth || false === $args ) {
+		return $classes;
+	}
+
 	// Max-depth is 1-based.
 	$max_depth = isset( $args->depth ) ? (int) $args->depth : 0;
 	// Depth is 0-based so needs to be increased by one.
diff --git a/tests/phpunit/tests/menu/wpNavMenuRemoveMenuItemHasChildrenClass.php b/tests/phpunit/tests/menu/wpNavMenuRemoveMenuItemHasChildrenClass.php
new file mode 100644
index 0000000000000..535eba068e3f0
--- /dev/null
+++ b/tests/phpunit/tests/menu/wpNavMenuRemoveMenuItemHasChildrenClass.php
@@ -0,0 +1,100 @@
+<?php
+
+/**
+ * @group menu
+ */
+class Tests_Menu_WpNavMenuRemoveMenuItemHasChildrenClass extends WP_UnitTestCase {
+
+	/**
+	 * Ensure calling filter in legacy ways does not throw an error.
+	 *
+	 * @ticket 56926
+	 */
+	public function test_legacy_filter_should_not_throw_an_error() {
+		$classes = array( 'menu-item-has-children', 'menu-item', 'menu-item-123' );
+
+		$menu_item = (object) array(
+			'classes' => $classes,
+		);
+
+		$args = (object) array(
+			'depth' => 2,
+		);
+
+		$depth = 2;
+
+		$class_names = implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $menu_item ) );
+		$this->assertStringContainsString( 'menu-item-has-children', $class_names, 'Class name should be retained when filter is called with two arguments.' );
+		$class_names = implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $menu_item, $args ) );
+		$this->assertStringContainsString( 'menu-item-has-children', $class_names, 'Class name should be retained when filter is called with three arguments.' );
+		$class_names = implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $menu_item, $args, $depth ) );
+		$this->assertStringNotContainsString( 'menu-item-has-children', $class_names, 'Class name should not be retained when filter is called with four arguments.' );
+	}
+
+	/**
+	 * Ensure menu-item-has-children class is removed or retained as expected.
+	 *
+	 * @dataProvider data_menu_item_has_children_class_should_be_removed_or_retained_as_expected
+	 * @ticket 56926
+	 */
+	public function test_menu_item_has_children_class_should_be_removed_or_retained_as_expected( $args, $depth, $should_be_retained ) {
+		$classes = array( 'menu-item-has-children', 'menu-item', 'menu-item-123' );
+
+		$menu_item = (object) array(
+			'classes' => $classes,
+		);
+
+		$class_names = wp_nav_menu_remove_menu_item_has_children_class( $classes, $menu_item, $args, $depth );
+		if ( $should_be_retained ) {
+			$this->assertContains( 'menu-item-has-children', $class_names, 'Class name should be retained.' );
+			return;
+		}
+
+		$this->assertNotContains( 'menu-item-has-children', $class_names, 'Class name should not be retained.' );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_menu_item_has_children_class_should_be_removed_or_retained_as_expected() {
+		return array(
+			'Depth not set'                          => array(
+				'args'               => (object) array( 'depth' => 1 ),
+				'depth'              => false,
+				'should_be_retained' => true,
+			),
+			'Neither depth nor args set'             => array(
+				'args'               => false,
+				'depth'              => false,
+				'should_be_retained' => true,
+			),
+			'Max depth is set to minus 1'            => array(
+				'args'               => (object) array( 'depth' => -1 ),
+				'depth'              => 1,
+				'should_be_retained' => false,
+			),
+			'Max depth is set to zero'               => array(
+				'args'               => (object) array( 'depth' => 0 ),
+				'depth'              => 1,
+				'should_be_retained' => true,
+			),
+			'Item depth exceeds max depth'           => array(
+				'args'               => (object) array( 'depth' => 2 ),
+				'depth'              => 3,
+				'should_be_retained' => false,
+			),
+			'Item depth is lower than max depth'     => array(
+				'args'               => (object) array( 'depth' => 5 ),
+				'depth'              => 3,
+				'should_be_retained' => true,
+			),
+			'Item depth is one lower than max depth' => array(
+				'args'               => (object) array( 'depth' => 2 ),
+				'depth'              => 1,
+				'should_be_retained' => false, // Depth is zero-based, max depth is not.
+			),
+		);
+	}
+}