Fatel Error on upgrade

Ive just updated my WP installation but am getting the following error when trying to access admin:

Fatal error: Call to undefined function wp_unslash() in …/public_html/wp-admin/menu.php on line 153

Looking at Line 153 it appars might be something to do with custom theme but thats a guess.

$customize_url = add_query_arg( 'return', urlencode( wp_unslash( $_SERVER['REQUEST_URI'] ) ), 'customize.php' );

The front-end of the site still works fine and shows the admin bar as im logged in, although icons are missing.

Line 150 to 170 is below

$menu[60] = array( __('Appearance'), $appearance_cap, 'themes.php', '', 'menu-top menu-icon-appearance', 'menu-appearance', 'dashicons-admin-appearance' );
	$submenu['themes.php'][5] = array( __( 'Themes' ), $appearance_cap, 'themes.php' );

	$customize_url = add_query_arg( 'return', urlencode( wp_unslash( $_SERVER['REQUEST_URI'] ) ), 'customize.php' );
	$submenu['themes.php'][6] = array( __( 'Customize' ), 'customize', $customize_url, '', 'hide-if-no-customize' );

	if ( current_theme_supports( 'menus' ) || current_theme_supports( 'widgets' ) ) {
		$submenu['themes.php'][10] = array(__( 'Menus' ), 'edit_theme_options', 'nav-menus.php');
	}

	if ( current_theme_supports( 'custom-header' ) && current_user_can( 'customize') ) {
		$customize_header_url = add_query_arg( 'autofocus[control]', 'header_image', $customize_url );
		$submenu['themes.php'][15] = array( __( 'Header' ), $appearance_cap, $customize_header_url, '', 'hide-if-no-customize' );
	}

	if ( current_theme_supports( 'custom-background' ) && current_user_can( 'customize') ) {
		$customize_background_url = add_query_arg( 'autofocus[control]', 'background_image', $customize_url );
		$submenu['themes.php'][20] = array( __( 'Background' ), $appearance_cap, $customize_background_url, '', 'hide-if-no-customize' );
	}

	unset( $customize_url );