diff --git a/make/py-defs.mk b/make/py-defs.mk
index a94a7edc..c3a7a76d 100644
--- a/make/py-defs.mk
+++ b/make/py-defs.mk
@@ -75,7 +75,9 @@ ifeq ($(PY_INSTALL_REG),true)
   PY_INSTALLED_REG           = $(PY_INSTALLED_PY) $(PY_INSTALLED_PYC)
 endif
 
-PYTHONPATH                  ?= $(call proj_query, pythonpath $(PROJECT) $(PY_PREREQ_RUN))
+ifeq ($(origin PYTHONPATH),undefined)
+  PYTHONPATH                := $(call proj_query, pythonpath $(PROJECT) $(PY_PREREQ_RUN))
+endif
 #PYTHONPATH                  = $(subst $(space),,$(ENV_PYTHONPATH)$(foreach d,$(PY_MODULE_DIRS),:$(d)))
 MYPYPATH                     = $(PYTHONPATH)
 export PYTHONPATH MYPYPATH