diff --git a/book/chapters/basics/hello-world/console2 b/book/chapters/basics/hello-world/console2
index e3e0d4e..2a4375b 100644
--- a/book/chapters/basics/hello-world/console2
+++ b/book/chapters/basics/hello-world/console2
@@ -1,4 +1,5 @@
 $ opam install ocamlfind
+$ opam install core
 $ ocamlfind opt -package core -thread -linkpkg hello2.ml
 $ ./a.out
 hello world