diff --git a/journal.md b/journal.md
index 30beea7..b5e3fb1 100644
--- a/journal.md
+++ b/journal.md
@@ -43,6 +43,7 @@ Reference 2 (1995).
 Reference 3 (1996).
 Reference 4 (2020).
 Reference 5 (1979).
+Reference 6 (1999).
 More missing references (2020)
 
 ## Appendix