diff --git a/Makefile b/Makefile
index 6c0635f69982..3c788a65d652 100644
--- a/Makefile
+++ b/Makefile
@@ -31,7 +31,7 @@ _all:
 # descending is started. They are now explicitly listed as the
 # prepare rule.
 
-ifneq ($(sub-make-done),1)
+ifneq ($(sub_make_done),1)
 
 # Do not use make's built-in rules and variables
 # (this increases performance and avoids hard-to-debug behaviour)
@@ -155,6 +155,8 @@ need-sub-make := 1
 $(lastword $(MAKEFILE_LIST)): ;
 endif
 
+export sub_make_done := 1
+
 ifeq ($(need-sub-make),1)
 
 PHONY += $(MAKECMDGOALS) sub-make
@@ -164,12 +166,12 @@ $(filter-out _all sub-make $(CURDIR)/Makefile, $(MAKECMDGOALS)) _all: sub-make
 
 # Invoke a second make in the output directory, passing relevant variables
 sub-make:
-	$(Q)$(MAKE) sub-make-done=1 \
+	$(Q)$(MAKE) \
 	$(if $(KBUILD_OUTPUT),-C $(KBUILD_OUTPUT) KBUILD_SRC=$(CURDIR)) \
 	-f $(CURDIR)/Makefile $(filter-out _all sub-make,$(MAKECMDGOALS))
 
 endif # need-sub-make
-endif # sub-make-done
+endif # sub_make_done
 
 # We process the rest of the Makefile if this is the final invocation of make
 ifeq ($(need-sub-make),)