diff --git a/make/py-defs.mk b/make/py-defs.mk
index 915491a2..de6da233 100644
--- a/make/py-defs.mk
+++ b/make/py-defs.mk
@@ -37,6 +37,8 @@ endif
 
 # deduce PY_INSTALL_DIR_PY from working directory below .. python/
 ifeq ($(PY_INSTALL_DIR_PY),)
+  ECHO                      ?= echo
+  SED                       ?= sed
   PY_INSTALL_PKG_MOD        ?= $(shell $(ECHO) $(CWD) | $(SED) 's%.*/python/%%; s%/.*%%')
   PY_INSTALL_SUB_MOD        ?= $(shell $(ECHO) $(CWD) | $(SED) "s%.*/$(PY_INSTALL_PKG_MOD)\(/\|$$\)%%")
   ifneq ($(PY_INSTALL_SUB_MOD),)