diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index a85422b831..a17b19e1b7 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -274,7 +274,7 @@ If you'd like to check if your links and formatting look good in JavaDoc (and no
 sbt -Dakka.genjavadoc.enabled=true javaunidoc:doc
 ```
 
-Which will generate JavaDoc style docs in `./target/javaunidoc/index.html`.
+Which will generate JavaDoc style docs in `./target/javaunidoc/index.html`. This requires a jdk version 9 or later.
 
 ## External dependencies