diff --git a/src/corelib/io/qfilesystemengine_win.cpp b/src/corelib/io/qfilesystemengine_win.cpp
index e4a7ea4891..5ed2671ab4 100644
--- a/src/corelib/io/qfilesystemengine_win.cpp
+++ b/src/corelib/io/qfilesystemengine_win.cpp
@@ -571,19 +571,21 @@ typedef struct _FILE_ID_INFO {
 // File ID for Windows up to version 7.
 static inline QByteArray fileId(HANDLE handle)
 {
-    QByteArray result;
 #ifndef Q_OS_WINRT
     BY_HANDLE_FILE_INFORMATION info;
     if (GetFileInformationByHandle(handle, &info)) {
-        result = QByteArray::number(uint(info.nFileIndexLow), 16);
-        result += ':';
-        result += QByteArray::number(uint(info.nFileIndexHigh), 16);
+        char buffer[sizeof "01234567:0123456701234567"];
+        qsnprintf(buffer, sizeof(buffer), "%lx:%08lx%08lx",
+                  info.dwVolumeSerialNumber,
+                  info.nFileIndexHigh,
+                  info.nFileIndexLow);
+        return buffer;
     }
 #else // !Q_OS_WINRT
     Q_UNUSED(handle);
     Q_UNIMPLEMENTED();
 #endif // Q_OS_WINRT
-    return result;
+    return QByteArray();
 }
 
 // File ID for Windows starting from version 8.