diff --git a/GNUmakefile b/GNUmakefile
index 865b35e8b..837f7103f 100755
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -2,7 +2,14 @@ SHELL=bash
 TMP_FOLDER=/tmp
 RELEASE_FOLDER=wllbg-release
 
-ENV ?= prod
+# ensure the ENV variable is well defined
+AVAILABLE_ENV := prod dev test
+ifneq ($(filter $(ENV),$(AVAILABLE_ENV)),)
+	# all good
+else
+	# not good, force it to "prod"
+	override ENV = prod
+endif
 
 help: ## Display this help menu
 	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'