diff --git a/scripts/common.mk b/scripts/common.mk
index 18e1813..ef894cb 100644
--- a/scripts/common.mk
+++ b/scripts/common.mk
@@ -495,8 +495,8 @@ build/fcos-dev.bu: SELF := $(PROJECT_NAME) $(PROJECT_NAME)-examples
 # The ignition files are needed to detect duplicates (see default-butane-spec.sh).
 build/fcos-dev.bu: build/$(PROJECT_NAME).ign build/$(PROJECT_NAME)-examples.ign $(DEPENDENCIES_IGNITION_EXAMPLES_FILES)
 build/fcos-test.bu: build/$(PROJECT_NAME).ign $(DEPENDENCIES_IGNITION_FILES)
-build/fcos-dev.bu build/fcos-test.bu: %.bu: Makefile $(SCRIPTS_DIR)/default-butane-spec.sh build
-	$(SCRIPTS_DIR)/default-butane-spec.sh $(foreach name,$(SELF) $(DEPS),$(call ignition_path,$(name))) > $@
+build/fcos-dev.bu build/fcos-test.bu: %.bu: Makefile $(SCRIPTS_DIR)/default-butane-spec.sh build $(TOP_LEVEL_DIR)/build/local.ign
+	$(SCRIPTS_DIR)/default-butane-spec.sh $(foreach name,$(SELF) $(DEPS),$(call ignition_path,$(name))) $(TOP_LEVEL_DIR)/build/local.ign > $@
 
 # Path of the ignition file of the current project or of one of its dependencies (with or without the "-examples" suffix).
 ignition_path = $(if $(filter $(PROJECT_NAME) $(PROJECT_NAME)-examples,$(1)),build/$(1).ign,$(COOKBOOKS_DIR)/$(patsubst %-examples,%,$(1))/build/$(1).ign)
diff --git a/scripts/default-butane-spec.sh b/scripts/default-butane-spec.sh
index a06a261..b5cbdb0 100755
--- a/scripts/default-butane-spec.sh
+++ b/scripts/default-butane-spec.sh
@@ -2,12 +2,12 @@
 
 set -Eeuo pipefail
 
-# Usage: default-butane-spec.sh <ignition file>...
+# Usage: default-butane-spec.sh <ignition file>... <local.ign>
 #
 # Butane inlines the "local" files as data URLs and rejects duplicate entries.
 # So, ignition files whose content is identical to a previous one (typically
-# the empty "-examples" ignition files) are skipped. Merging the same config
-# twice would be a no-op anyway.
+# the empty "-examples" ignition files, or an empty local.ign) are skipped.
+# Merging the same config twice would be a no-op anyway.
 
 cat <<'EOF'
 variant: fcos
@@ -26,4 +26,3 @@ for ign in "$@"; do
     seen[$checksum]="$(basename "$ign")"
     echo "    - local: $(basename "$ign")"
 done
-echo "    - local: local.ign"