diff --git a/src/bmffimage.cpp b/src/bmffimage.cpp
index 6c6c472e1e..3d7c23e936 100644
--- a/src/bmffimage.cpp
+++ b/src/bmffimage.cpp
@@ -212,7 +212,7 @@ namespace Exiv2
         if ( bTrace ) {
             bLF = true;
             out << indent(depth) << "Exiv2::BmffImage::boxHandler: " << toAscii(box_type)
-                << Internal::stringFormat(" %8ld->%u ", address, box_length);
+                << Internal::stringFormat(" %8ld->%lu ", address, box_length);
         }
 
         if (box_length == 1) {