lib/PublicInbox/ExtSearchIdx.pm | 4 +++-

diff --git a/lib/PublicInbox/ExtSearchIdx.pm b/lib/PublicInbox/ExtSearchIdx.pm
index 2311161eca99d7a3a3da348b400bc33b38224651..1c2a97588e3432960d87ad50390f20bf35317adf 100644
--- a/lib/PublicInbox/ExtSearchIdx.pm
+++ b/lib/PublicInbox/ExtSearchIdx.pm
@@ -937,7 +937,9 @@ 				self => $self,
 			};
 			$self->git->cat_async($smsg->{blob}, \&dd_smsg, $dd);
 		}
-		# need to wait on every single one
+		# need to wait on every single one @smsg contents can get
+		# invalidated inside dd_smsg for messages with multiple
+		# Message-IDs.
 		$self->git->async_wait_all;
 
 		if (checkpoint_due($sync)) {