diff --git a/toonz/sources/common/tsystem/tsystem.cpp b/toonz/sources/common/tsystem/tsystem.cpp
index 652a8b1..502b967 100644
--- a/toonz/sources/common/tsystem/tsystem.cpp
+++ b/toonz/sources/common/tsystem/tsystem.cpp
@@ -462,7 +462,7 @@ void TSystem::readDirectory_DirItems(QStringList &dst, const TFilePath &path) {
 #ifdef _WIN32
   // equivalent to sorting with QDir::LocaleAware
   struct strCompare {
-    bool operator()(const QString &s1, const QString &s2) {
+    bool operator()(const QString &s1, const QString &s2) const {
       return QString::localeAwareCompare(s1, s2) < 0;
     }
   };