diff --git a/ddclient.in b/ddclient.in
index 7d244db..d8fbbf8 100755
--- a/ddclient.in
+++ b/ddclient.in
@@ -3381,7 +3381,7 @@ sub group_hosts_by {
     my %groups;
     my %cfgs;
     for my $h (@$hosts) {
-        my %cfg = map({ ($_ => $config{$h}{$_}); } grep(exists($config{$h}{$_}), @attrs));
+        my %cfg = map({ ($_ => $config{$h}{$_}); } grep(defined($config{$h}{$_}), @attrs));
         my $sig = repr(\%cfg, Indent => 0);
         push(@{$groups{$sig}}, $h);
         $cfgs{$sig} = \%cfg;
diff --git a/t/group_hosts_by.pl b/t/group_hosts_by.pl
index 4e2c29f..4cf25a1 100644
--- a/t/group_hosts_by.pl
+++ b/t/group_hosts_by.pl
@@ -72,7 +72,9 @@ my @test_cases = (
         want => [
             {cfg => {falsy => 0}, hosts => [$h1]},
             {cfg => {falsy => ''}, hosts => [$h2]},
-            {cfg => {falsy => undef}, hosts => [$h3]},
+            # undef intentionally becomes unset because undef always means "fall back to global or
+            # default".
+            {cfg => {}, hosts => [$h3]},
         ],
     },
     {
@@ -80,8 +82,9 @@ my @test_cases = (
         groupby => [qw(maybeunset)],
         want => [
             {cfg => {maybeunset => 'unique'}, hosts => [$h1]},
-            {cfg => {maybeunset => undef}, hosts => [$h2]},
-            {cfg => {}, hosts => [$h3]},
+            # undef intentionally becomes unset because undef always means "fall back to global or
+            # default".
+            {cfg => {}, hosts => [$h2, $h3]},
         ],
     },
     {