diff --git a/include/library.h b/include/library.h
index 7c806ed6d..fd5816882 100644
--- a/include/library.h
+++ b/include/library.h
@@ -35,6 +35,7 @@ namespace kiwix
 {
 
 class OPDSDumper;
+class Library;
 
 enum supportedListSortBy { UNSORTED, TITLE, SIZE, DATE, CREATOR, PUBLISHER };
 enum supportedListMode {
@@ -110,6 +111,9 @@ class Filter {
     const std::string& getQuery() const { return _query; }
     bool queryIsPartial() const { return _queryIsPartial; }
 
+private:
+    friend class Library;
+
     bool accept(const Book& book) const;
     bool acceptByQueryOnly(const Book& book) const;
     bool acceptByNonQueryCriteria(const Book& book) const;