diff --git a/wp-content/themes/twentyfifteen/functions.php b/wp-content/themes/twentyfifteen/functions.php
index cc0d81639c..b560e83de0 100644
--- a/wp-content/themes/twentyfifteen/functions.php
+++ b/wp-content/themes/twentyfifteen/functions.php
@@ -34,6 +34,13 @@ if ( ! isset( $content_width ) ) {
 	$content_width = 660;
 }
 
+/**
+ * Twenty Fifteen only works in WordPress 4.1 or later.
+ */
+if ( version_compare( $GLOBALS['wp_version'], '4.1', '<' ) ) {
+	require get_template_directory() . '/inc/back-compat.php';
+}
+
 if ( ! function_exists( 'twentyfifteen_setup' ) ) :
 /**
  * Sets up theme defaults and registers support for various WordPress features.
diff --git a/wp-content/themes/twentyfifteen/inc/back-compat.php b/wp-content/themes/twentyfifteen/inc/back-compat.php
new file mode 100644
index 0000000000..0536c0103c
--- /dev/null
+++ b/wp-content/themes/twentyfifteen/inc/back-compat.php
@@ -0,0 +1,63 @@
+<?php
+/**
+ * Twenty Fifteen back compat functionality
+ *
+ * Prevents Twenty Fifteen from running on WordPress versions prior to 4.1,
+ * since this theme is not meant to be backward compatible beyond that
+ * and relies on many newer functions and markup changes introduced in 4.1.
+ *
+ * @package WordPress
+ * @subpackage Twenty_Fifteen
+ * @since Twenty Fifteen 1.0
+ */
+
+/**
+ * Prevent switching to Twenty Fifteen on old versions of WordPress.
+ *
+ * Switches to the default theme.
+ *
+ * @since Twenty Fifteen 1.0
+ */
+function twentyfifteen_switch_theme() {
+	switch_theme( WP_DEFAULT_THEME, WP_DEFAULT_THEME );
+	unset( $_GET['activated'] );
+	add_action( 'admin_notices', 'twentyfifteen_upgrade_notice' );
+}
+add_action( 'after_switch_theme', 'twentyfifteen_switch_theme' );
+
+/**
+ * Add message for unsuccessful theme switch.
+ *
+ * Prints an update nag after an unsuccessful attempt to switch to
+ * Twenty Fifteen on WordPress versions prior to 4.1.
+ *
+ * @since Twenty Fifteen 1.0
+ */
+function twentyfifteen_upgrade_notice() {
+	$message = sprintf( __( 'Twenty Fifteen requires at least WordPress version 4.1. You are running version %s. Please upgrade and try again.', 'twentyfifteen' ), $GLOBALS['wp_version'] );
+	printf( '<div class="error"><p>%s</p></div>', $message );
+}
+
+/**
+ * Prevent the Customizer from being loaded on WordPress versions prior to 4.1.
+ *
+ * @since Twenty Fifteen 1.0
+ */
+function twentyfifteen_customize() {
+	wp_die( sprintf( __( 'Twenty Fifteen requires at least WordPress version 4.1. You are running version %s. Please upgrade and try again.', 'twentyfifteen' ), $GLOBALS['wp_version'] ), '', array(
+		'back_link' => true,
+	) );
+}
+add_action( 'load-customize.php', 'twentyfifteen_customize' );
+
+/**
+ * Prevent the Theme Preview from being loaded on WordPress versions prior to 4.1.
+ *
+ * @since Twenty Fifteen 1.0
+ */
+function twentyfifteen_preview() {
+	if ( isset( $_GET['preview'] ) ) {
+		wp_die( sprintf( __( 'Twenty Fifteen requires at least WordPress version 4.1. You are running version %s. Please upgrade and try again.', 'twentyfifteen' ), $GLOBALS['wp_version'] ) );
+	}
+}
+add_action( 'template_redirect', 'twentyfifteen_preview' );
\ No newline at end of file
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 98d2966eec..1c14ea9beb 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.1-alpha-30098';
+$wp_version = '4.1-alpha-30099';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.