lib/PublicInbox/LeiMirror.pm | 5 +++--

diff --git a/lib/PublicInbox/LeiMirror.pm b/lib/PublicInbox/LeiMirror.pm
index ddb1e74794102761beb7f5a9f65757f0123ba550..0f46d35531f890ec9bacba93c7ea6f64d336c27c 100644
--- a/lib/PublicInbox/LeiMirror.pm
+++ b/lib/PublicInbox/LeiMirror.pm
@@ -539,8 +539,9 @@ 	# resolve references, deepest, first:
 	while (scalar keys %$todo) {
 		for my $x (keys %$todo) {
 			# resolve multi-level references
-			while (defined($m->{$x}->{reference})) {
-				$x = $m->{$x}->{reference};
+			while (defined(my $nxt = $m->{$x}->{reference})) {
+				exists($todo->{$nxt}) or last;
+				$x = $nxt;
 			}
 			my $y = delete $todo->{$x} // next; # already done
 			for (@$y) {