diff --git a/Makefile b/Makefile
index 7db1c6d..729f13d 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,13 @@ export PYTHONPATH := $(PWD)/tools/nrfutil:$(PWD)/tools/intelhex:$(PYTHONPATH)
 
 all : bootloader reloader micropython
 
+ifdef BOARD
+WASP_WATCH_PY = wasp/boards/$(BOARD)/watch.py
+$(WASP_WATCH_PY) : $(WASP_WATCH_PY).in
+	(cd wasp; ../tools/preprocess.py $(WASP_WATCH_PY).in $(WASP_WATCH_PY))
+else
 BOARD ?= $(error Please set BOARD=)
+endif
 
 clean :
 	$(RM) -r \
@@ -31,7 +37,7 @@ reloader: bootloader
 softdevice:
 	micropython/ports/nrf/drivers/bluetooth/download_ble_stack.sh
 
-micropython: wasp/boards/$(BOARD)/watch.py
+micropython: $(WASP_WATCH_PY)
 	$(MAKE) -C micropython/mpy-cross
 	$(RM) micropython/ports/nrf/build-$(BOARD)-s132/frozen_content.c
 	$(MAKE) -C micropython/ports/nrf \
@@ -44,10 +50,6 @@ micropython: wasp/boards/$(BOARD)/watch.py
 		--application micropython/ports/nrf/build-$(BOARD)-s132/firmware.hex \
 		micropython.zip
 
-wasp/boards/$(BOARD)/watch.py : wasp/boards/$(BOARD)/watch.py.in
-	(cd wasp; ../tools/preprocess.py boards/$(BOARD)/watch.py.in > \
-		                         boards/$(BOARD)/watch.py)
-
 dfu:
 	python3 -m nordicsemi dfu serial --package micropython.zip --port /dev/ttyACM0