diff --git a/IkiWiki/Plugin/comments.pm b/IkiWiki/Plugin/comments.pm
index 6bd18a5cf..d1105b366 100644
--- a/IkiWiki/Plugin/comments.pm
+++ b/IkiWiki/Plugin/comments.pm
@@ -434,8 +434,10 @@ sub sessioncgi ($$) { #{{{
 	}
 
 	my $subject = $form->field('subject');
-	$subject =~ s/"/"/g;
-	$content .= " subject=\"$subject\"\n";
+	if (length $subject) {
+		$subject =~ s/"/"/g;
+		$content .= " subject=\"$subject\"\n";
+	}
 
 	$content .= " date=\"" . IkiWiki::formattime(time, '%X %x') . "\"\n";