diff --git a/vncviewer/X11PixelBuffer.cxx b/vncviewer/X11PixelBuffer.cxx
index 3c41490..ce5c4d8 100644
--- a/vncviewer/X11PixelBuffer.cxx
+++ b/vncviewer/X11PixelBuffer.cxx
@@ -252,7 +252,7 @@
   xevent = (XEvent*)event;
   assert(xevent);
 
-  if (xevent->type != XShmGetEventBase(fl_display))
+  if (xevent->type != XShmGetEventBase(fl_display) + ShmCompletion)
     return 0;
 
   shmevent = (XShmCompletionEvent*)event;