diff --git a/lib/grandpa/message_tracker.go b/lib/grandpa/message_tracker.go
index 26bb1b42c2..00e7ef801a 100644
--- a/lib/grandpa/message_tracker.go
+++ b/lib/grandpa/message_tracker.go
@@ -113,6 +113,7 @@ func (t *tracker) handleBlock(b *types.Block) {
 		}
 	}
 
+	// delete block hash that may or may not be in the tracker.
 	t.votes.delete(h)
 
 	if cm, has := t.commitMessages[h]; has {