diff --git a/src/emacs/Cask b/src/emacs/Cask
index eebe3d2d81..1df55fff1d 100644
--- a/src/emacs/Cask
+++ b/src/emacs/Cask
@@ -1,3 +1,4 @@
+(source gnu)
 (source melpa)
 (package-file "lean-mode.el")
 (development