diff --git a/message.go b/message.go
index 57520f5..c5b93dc 100644
--- a/message.go
+++ b/message.go
@@ -131,6 +131,9 @@ func ParseSlack(b []byte) (Message, error) {
 		}, nil
 	}
 
+	if s.Event.ParentID == "" {
+		return Message{}, ErrIrrelevantMessage
+	}
 	return Message{
 		ID:        fmt.Sprintf("%s/%v", s.Event.ParentID, s.TS),
 		TS:        s.TS,