diff --git a/mk/px4_targets.mk b/mk/px4_targets.mk
index 8a5446d4c3..6bebd6b7d5 100644
--- a/mk/px4_targets.mk
+++ b/mk/px4_targets.mk
@@ -11,4 +11,9 @@ px4-clean:
 px4-upload:
 	make -C $(PX4_ROOT) EXTERNAL_APPS=$(PWD) upload
 
+else
+
+px4:
+	$(error ERROR: You need to add PX4_ROOT to your config.mk)
+
 endif