Index: wp-content/themes/twentytwelve/js/theme.js
===================================================================
--- wp-content/themes/twentytwelve/js/theme.js	(revision 21387)
+++ wp-content/themes/twentytwelve/js/theme.js	(working copy)
@@ -6,7 +6,7 @@
 
 jQuery( document ).ready( function( $ ) {
 	var masthead = $( '#masthead' ),
-		largeWindow = window.matchMedia( 'screen and (min-width: 600px)' ),
+		largeWindow = ( ! window.matchMedia ) || window.matchMedia( 'screen and (min-width: 600px)' ).matches,
 		timeout = false;
 
 	$.fn.smallMenu = function() {
@@ -20,7 +20,7 @@
 	};
 
 	// Check viewport width on first load.
-	if ( ! largeWindow.matches )
+	if ( ! largeWindow )
 		$.fn.smallMenu();
 
 	// Check viewport width when user resizes the browser window.
@@ -29,7 +29,7 @@
 			clearTimeout( timeout );
 
 		timeout = setTimeout( function() {
-			if ( ! largeWindow.matches ) {
+			if ( ! largeWindow ) {
 				$.fn.smallMenu();
 			} else {
 				masthead.find( '.site-navigation' ).removeClass( 'main-small-navigation' ).addClass( 'main-navigation' );