--- a/lib/DNStools/UpdateSerial.pm	Mon Jun 27 15:06:30 2011 +0200
+++ b/lib/DNStools/UpdateSerial.pm	Tue Jun 28 14:13:49 2011 +0200
@@ -330,6 +330,7 @@
         chmod $mode, $cb or die "Can't 'chmod $mode, $cb': $!";
         utime $atime, $mtime, $cb or die "Can't 'utime $atime, $mtime, $cb': $!";
 
+        truncate C, 0 or die "Can't truncate C, 0: $!";
         seek C, 0, 0 or die "Can't seek C, 0, 0: $!";
         # write back @lines we modified earlier
         print C @lines;