diff --git a/mpy-cross/Makefile b/mpy-cross/Makefile
index 6ed4e653f8..de07d2f6d4 100644
--- a/mpy-cross/Makefile
+++ b/mpy-cross/Makefile
@@ -1,3 +1,14 @@
+# The following is a temporary hack to forefully undefine vars that might have
+# be defined by a calling Makefile (from recursive make).
+# TODO: Find a better way to be able to call this Makefile recursively.
+override undefine COPT
+override undefine CFLAGS_EXTRA
+override undefine LDFLAGS_EXTRA
+override undefine FROZEN_DIR
+override undefine FROZEN_MPY_DIR
+override undefine BUILD
+override undefine PROG
+
 include ../py/mkenv.mk
 
 # define main target