diff --git a/lib/class-wp-rest-sidebars-controller.php b/lib/class-wp-rest-sidebars-controller.php
index 971a7dad4a7bfa..d5715ca5587ba6 100644
--- a/lib/class-wp-rest-sidebars-controller.php
+++ b/lib/class-wp-rest-sidebars-controller.php
@@ -142,7 +142,7 @@ public function update_item( $request ) {
 		$sidebar_widgets_ids = array();
 		foreach ( $input_widgets as $input_widget ) {
 			ob_start();
-			if ( isset( $wp_registered_widget_updates[ $input_widget['id_base'] ] ) ) {
+			if ( isset( $input_widget['id_base'] ) && isset( $wp_registered_widget_updates[ $input_widget['id_base'] ] ) ) {
 				// Class-based widget.
 				$update_control = $wp_registered_widget_updates[ $input_widget['id_base'] ];
 				if ( ! isset( $input_widget['id'] ) ) {
@@ -176,11 +176,23 @@ public function update_item( $request ) {
 						$wp_registered_widgets[ $input_widget['id'] ]['callback'][0] = $new_object;
 					}
 				}
-			} elseif ( $wp_registered_widget_updates[ $input_widget['id'] ] ) {
-				// Old-style widget.
-				$update_control = $wp_registered_widget_updates[ $input_widget['id'] ];
-				$_POST          = wp_slash( $input_widget['settings'] );
-				call_user_func( $update_control['callback'] );
+			} else {
+				$registered_widget_id = null;
+				if ( isset( $wp_registered_widget_updates[ $input_widget['id'] ] ) ) {
+					$registered_widget_id = $input_widget['id'];
+				} else {
+					$numberless_id = substr( $input_widget['id'], 0, strrpos( $input_widget['id'], '-' ) );
+					if ( isset( $wp_registered_widget_updates[ $numberless_id ] ) ) {
+						$registered_widget_id = $numberless_id;
+					}
+				}
+
+				if ( $registered_widget_id ) {
+					// Old-style widget.
+					$update_control = $wp_registered_widget_updates[ $registered_widget_id ];
+					$_POST          = wp_slash( $input_widget['settings'] );
+					call_user_func( $update_control['callback'] );
+				}
 			}
 			ob_end_clean();
 
@@ -213,6 +225,7 @@ public function get_items( $request ) {
 
 			$data[] = $this->prepare_item_for_response( $sidebar, $request )->get_data();
 		}
+
 		return rest_ensure_response( $data );
 	}
 
@@ -350,7 +363,7 @@ public static function get_widgets( $sidebar_id ) {
 	 * Prepare a single sidebar output for response
 	 *
 	 * @param array           $raw_sidebar Sidebar instance.
-	 * @param WP_REST_Request $request     Request object.
+	 * @param WP_REST_Request $request Request object.
 	 *
 	 * @return WP_REST_Response $data
 	 */
@@ -390,7 +403,7 @@ public function prepare_item_for_response( $raw_sidebar, $request ) {
 			foreach ( $schema['properties']['widgets']['items']['properties'] as $property_id => $property ) {
 				if ( isset( $widget[ $property_id ] ) && gettype( $widget[ $property_id ] ) === $property['type'] ) {
 					$widget_data[ $property_id ] = $widget[ $property_id ];
-				} elseif ( 'settings' === $property_id && 'array' === gettype( $widget[ $property_id ] ) ) {
+				} elseif ( 'settings' === $property_id && isset( $widget[ $property_id ] ) && 'array' === gettype( $widget[ $property_id ] ) ) {
 					$widget_data[ $property_id ] = $widget['settings'];
 				} elseif ( isset( $property['default'] ) ) {
 					$widget_data[ $property_id ] = $property['default'];
diff --git a/phpunit/class-rest-sidebars-controller-test.php b/phpunit/class-rest-sidebars-controller-test.php
index cfd9dfa85392bc..8059cb2fcb4da1 100644
--- a/phpunit/class-rest-sidebars-controller-test.php
+++ b/phpunit/class-rest-sidebars-controller-test.php
@@ -378,6 +378,98 @@ public function test_update_item() {
 		);
 	}
 
+	/**
+	 *
+	 */
+	public function test_update_item_legacy_widget_1() {
+		$this->do_test_update_item_legacy_widget( 'testwidget-1' );
+	}
+
+	/**
+	 *
+	 */
+	public function test_update_item_legacy_widget_2() {
+		$this->do_test_update_item_legacy_widget( 'testwidget' );
+	}
+
+	/**
+	 *
+	 */
+	public function do_test_update_item_legacy_widget( $widget_id ) {
+		// @TODO: Use @dataProvider instead (it doesn't work with custom constructors like the one we have in this class)
+		wp_register_widget_control(
+			$widget_id,
+			'WP test widget',
+			function() {
+				$settings = get_option( 'widget_testwidget' );
+
+				// check if anything's been sent.
+				if ( isset( $_POST['update_testwidget'] ) ) {
+					$settings['id']    = $_POST['test_id'];
+					$settings['title'] = $_POST['test_title'];
+
+					update_option( 'widget_testwidget', $settings );
+				}
+			},
+			100,
+			200
+		);
+		wp_register_sidebar_widget(
+			$widget_id,
+			'WP test widget',
+			function() {
+				$settings = get_option( 'widget_testwidget' ) ? get_option( 'widget_testwidget' ) : array(
+					'id'    => '',
+					'title' => '',
+				);
+				echo '<h1>' . $settings['id'] . '</h1><span>' . $settings['title'] . '</span>';
+			}
+		);
+		$this->setup_sidebar(
+			'sidebar-1',
+			array(
+				'name' => 'Test sidebar',
+			),
+			array( $widget_id )
+		);
+
+		$request = new WP_REST_Request( 'POST', '/__experimental/sidebars/sidebar-1' );
+		$request->set_body_params(
+			array(
+				'widgets' => array(
+					array(
+						'id'       => $widget_id,
+						'name'     => 'WP test widget',
+						'settings' => array(
+							'test_id'           => 'My test id',
+							'test_title'        => 'My test title',
+							'update_testwidget' => true,
+						),
+					),
+				),
+			)
+		);
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		$this->assertEquals(
+			array(
+				'id'          => 'sidebar-1',
+				'name'        => 'Test sidebar',
+				'description' => '',
+				'status'      => 'active',
+				'widgets'     => array(
+					array(
+						'id'       => $widget_id,
+						'settings' => array(),
+						'rendered' => '<h1>My test id</h1><span>My test title</span>',
+						'name'     => 'WP test widget',
+					),
+				),
+			),
+			$data
+		);
+	}
+
 	/**
 	 * The test_delete_item() method does not exist for sidebar.
 	 */