diff --git a/py/py.mk b/py/py.mk
index e352d89792..e7716a1adc 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -252,7 +252,7 @@ $(HEADER_BUILD)/compressed.data.h: $(HEADER_BUILD)/compressed.collected
 	$(Q)$(PYTHON) $(PY_SRC)/makecompresseddata.py $< > $@
 
 # build a list of registered modules for py/objmodule.c.
-$(HEADER_BUILD)/moduledefs.h: $(HEADER_BUILD)/moduledefs.collected
+$(HEADER_BUILD)/moduledefs.h: $(HEADER_BUILD)/moduledefs.collected $(PY_SRC)/makemoduledefs.py
 	@$(ECHO) "GEN $@"
 	$(Q)$(PYTHON) $(PY_SRC)/makemoduledefs.py $< > $@