diff --git a/myhdl/_traceSignals.py b/myhdl/_traceSignals.py
index 20f314d3..49b3002e 100644
--- a/myhdl/_traceSignals.py
+++ b/myhdl/_traceSignals.py
@@ -125,7 +125,7 @@ class _TraceSignalsClass(object):
 
             if path.exists(vcdpath):
                 if self.tracebackup :
-                    backup = vcdpath + '.' + str(path.getmtime(vcdpath))
+                    backup = vcdpath[:-4] + '.' + str(path.getmtime(vcdpath)) + '.vcd'
                     shutil.copyfile(vcdpath, backup)
                 os.remove(vcdpath)
             vcdfile = open(vcdpath, 'w')