diff --git a/OPDS_renderer.php b/OPDS_renderer.php
index 5a05e88..00fdeb1 100644
--- a/OPDS_renderer.php
+++ b/OPDS_renderer.php
@@ -133,7 +133,7 @@ class OPDSRenderer
             if ($page->containsBook () && !is_null ($config['cops_books_filter']) && count ($config['cops_books_filter']) > 0) {
                 $Urlfilter = getURLParam ("tag", "");
                 foreach ($config['cops_books_filter'] as $lib => $filter) {
-                    $link = new LinkFacet ("?" . $_SERVER['QUERY_STRING'] . "&tag=" . $filter, $lib, localize ("tagword.title"), $filter == $Urlfilter);
+                    $link = new LinkFacet ("?" . addURLParameter ($_SERVER['QUERY_STRING'], "tag", $filter), $lib, localize ("tagword.title"), $filter == $Urlfilter);
                     self::renderLink ($link);
                 }
             }
diff --git a/base.php b/base.php
index ae4e559..470a62c 100644
--- a/base.php
+++ b/base.php
@@ -81,6 +81,17 @@ function localize($phrase, $count=-1) {
     return $translations[$phrase];
 }
 
+function addURLParameter($urlParams, $paramName, $paramValue) {
+    $params = array();
+    parse_str($urlParams, $params);
+    if (empty ($paramValue)) {
+        unset ($params[$paramName]);
+    } else {
+        $params[$paramName] = $paramValue;   
+    }
+    return http_build_query($params);
+}
+
 class Link
 {
     const OPDS_THUMBNAIL_TYPE = "http://opds-spec.org/image/thumbnail";
diff --git a/book.php b/book.php
index fd62e12..6832643 100644
--- a/book.php
+++ b/book.php
@@ -119,7 +119,7 @@ class Book extends Base {
     
     public function getFilterString () {
         $filter = getURLParam ("tag", NULL);
-        if (is_null ($filter)) return "";
+        if (empty ($filter)) return "";
         
         $exists = true;
         if (preg_match ("/^!(.*)$/", $filter, $matches)) {
diff --git a/config_default.php b/config_default.php
index 247fed9..4cb57ee 100644
--- a/config_default.php
+++ b/config_default.php
@@ -139,7 +139,7 @@
     /*
      * Filter on tags to book list
      */
-    $config['cops_books_filter'] = array ("Non lus" => "!Read", "lus" => "Read");  
+    $config['cops_books_filter'] = array ("Tout" => "", "Non lus" => "!Read", "lus" => "Read");  
     
     /*
      * Custom Columns to add  as an array containing the lookup names