diff --git a/Makefile b/Makefile
index 9d633ce486d..88cb0a2ab61 100644
--- a/Makefile
+++ b/Makefile
@@ -361,8 +361,8 @@ promote:
 	$(dune) promote $(ws_main)
 
 .PHONY: fmt
-fmt: duneconf/main.ws
-	$(dune) build $(ws_main) @fmt --auto-promote
+fmt: duneconf/boot.ws ocaml/dirs-to-ignore.inc
+	$(dune) build $(ws_boot) @fmt --auto-promote
 
 .PHONY: check-fmt
 check-fmt: