diff --git a/Makefile b/Makefile
index ce5540d..828b76f 100644
--- a/Makefile
+++ b/Makefile
@@ -36,5 +36,5 @@ lib/%: .FORCE
 bootstrap:
 	git submodule init
 	git submodule update
-	git submodule foreach 'git checkout master; git reset --hard $sha1'
+	git submodule foreach 'git checkout master; git reset --hard $$sha1'
 	make