diff --git a/vncviewer/Viewport.cxx b/vncviewer/Viewport.cxx
index 6c77e58..3d6f4a8 100644
--- a/vncviewer/Viewport.cxx
+++ b/vncviewer/Viewport.cxx
@@ -351,7 +351,7 @@
                      Fl::event_length() + 1);
     assert(ret < (Fl::event_length() + 1));
 
-    vlog.debug("Sending clipboard data (%d bytes)", strlen(buffer));
+    vlog.debug("Sending clipboard data (%d bytes)", (int)strlen(buffer));
 
     try {
       cc->writer()->clientCutText(buffer, ret);