diff --git a/src/PVE/SectionConfig.pm b/src/PVE/SectionConfig.pm
index 09ab5c2..5ddb382 100644
--- a/src/PVE/SectionConfig.pm
+++ b/src/PVE/SectionConfig.pm
@@ -381,7 +381,7 @@ sub check_config {
 	my $value = $config->{$k};
 
 	die "can't change value of fixed parameter '$k'\n"
-	    if !$create && $opts->{$k} && $opts->{$k}->{fixed};
+	    if !$create && defined($opts->{$k}) && $opts->{$k}->{fixed};
 
 	if (defined($value)) {
 	    my $tmp = $class->check_value($type, $k, $value, $sectionId, $skipSchemaCheck);