diff --git a/lib/csslib.php b/lib/csslib.php
index 974ae4a1d0950..6bf15785fe936 100644
--- a/lib/csslib.php
+++ b/lib/csslib.php
@@ -228,7 +228,7 @@ function css_send_ie_css($themename, $rev, $etag, $slasharguments) {
         $css .= "\n@import url($relroot/styles.php?theme=$themename&rev=$rev&type=theme);";
     }
 
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
     header('Content-Disposition: inline; filename="styles.php"');
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', time()) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
@@ -254,7 +254,7 @@ function css_send_ie_css($themename, $rev, $etag, $slasharguments) {
 function css_send_cached_css($csspath, $etag) {
     $lifetime = 60*60*24*60; // 60 days only - the revision may get incremented quite often
 
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
     header('Content-Disposition: inline; filename="styles.php"');
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($csspath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
@@ -312,7 +312,7 @@ function css_send_unmodified($lastmodified, $etag) {
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Cache-Control: public, max-age='.$lifetime);
     header('Content-Type: text/css; charset=utf-8');
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
     if ($lastmodified) {
         header('Last-Modified: '. gmdate('D, d M Y H:i:s', $lastmodified) .' GMT');
     }
diff --git a/lib/jslib.php b/lib/jslib.php
index 1d20bbd3b640f..bbfb895e5348a 100644
--- a/lib/jslib.php
+++ b/lib/jslib.php
@@ -36,7 +36,7 @@ function js_send_cached($jspath, $etag, $filename = 'javascript.php') {
 
     $lifetime = 60*60*24*60; // 60 days only - the revision may get incremented quite often
 
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
     header('Content-Disposition: inline; filename="'.$filename.'"');
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($jspath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
@@ -86,7 +86,7 @@ function js_send_unmodified($lastmodified, $etag) {
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Cache-Control: public, max-age='.$lifetime);
     header('Content-Type: application/javascript; charset=utf-8');
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
     if ($lastmodified) {
         header('Last-Modified: '. gmdate('D, d M Y H:i:s', $lastmodified) .' GMT');
     }
diff --git a/theme/image.php b/theme/image.php
index 75b7c599617bc..fc9402a5c2702 100644
--- a/theme/image.php
+++ b/theme/image.php
@@ -114,7 +114,7 @@
             header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
             header('Cache-Control: public, max-age='.$lifetime);
             header('Content-Type: '.$mimetype);
-            header('Etag: '.$etag);
+            header('Etag: "'.$etag.'"');
             die;
         }
         send_cached_image($cacheimage, $etag);
@@ -206,7 +206,7 @@ function send_cached_image($imagepath, $etag) {
 
     $mimetype = get_contenttype_from_ext($pathinfo['extension']);
 
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
     header('Content-Disposition: inline; filename="'.$imagename.'"');
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($imagepath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
@@ -293,4 +293,4 @@ function cache_image($image, $imagefile, $candidatelocation) {
         @unlink($cacheimage.'.tmp'); // just in case anything fails
     }
     return $cacheimage;
-}
\ No newline at end of file
+}
diff --git a/theme/yui_combo.php b/theme/yui_combo.php
index bd19b5039cc26..53b0e7f78a819 100644
--- a/theme/yui_combo.php
+++ b/theme/yui_combo.php
@@ -61,7 +61,7 @@
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Cache-Control: public, max-age='.$lifetime);
     header('Content-Type: '.$mimetype);
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
     die;
 }
 
@@ -187,7 +187,7 @@ function combo_send_cached($content, $mimetype, $etag, $lastmodified) {
     header('Cache-Control: public, max-age='.$lifetime);
     header('Accept-Ranges: none');
     header('Content-Type: '.$mimetype);
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
     if (!min_enable_zlib_compression()) {
         header('Content-Length: '.strlen($content));
     }
diff --git a/theme/yui_image.php b/theme/yui_image.php
index 4477a9c36c073..8d3436ec2b459 100644
--- a/theme/yui_image.php
+++ b/theme/yui_image.php
@@ -92,7 +92,7 @@
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Cache-Control: public, max-age='.$lifetime);
     header('Content-Type: '.$mimetype);
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
     die;
 }
 
@@ -113,7 +113,7 @@ function yui_image_cached($imagepath, $imagename, $mimetype, $etag) {
     header('Accept-Ranges: none');
     header('Content-Type: '.$mimetype);
     header('Content-Length: '.filesize($imagepath));
-    header('Etag: '.$etag);
+    header('Etag: "'.$etag.'"');
 
     if (xsendfile($imagepath)) {
         die;