diff --git a/perl-xCAT/xCAT/NetworkUtils.pm b/perl-xCAT/xCAT/NetworkUtils.pm
index 9a0fbef1d..89a293472 100644
--- a/perl-xCAT/xCAT/NetworkUtils.pm
+++ b/perl-xCAT/xCAT/NetworkUtils.pm
@@ -1389,6 +1389,7 @@ sub formatNetmask
     my $origType = shift;
     my $newType  = shift;
     my $maskn;
+    if (not defined $mask) { return undef; }
     if ($origType == 0)
     {
         $maskn = unpack("N", inet_aton($mask));