diff --git a/src/generic/htmllbox.cpp b/src/generic/htmllbox.cpp
index e0b3fd08f1..b104f3c2f5 100644
--- a/src/generic/htmllbox.cpp
+++ b/src/generic/htmllbox.cpp
@@ -301,7 +301,7 @@ void wxHtmlListBox::CacheItem(size_t n) const
 
         // set the cell's ID to item's index so that CellCoordsToPhysical()
         // can quickly find the item:
-        cell->SetId(wxString::Format(_T("%u"), n));
+        cell->SetId(wxString::Format(_T("%lu"), (unsigned long)n));
 
         cell->Layout(GetClientSize().x - 2*GetMargins().x);