diff --git a/py/mkrules.mk b/py/mkrules.mk
index b37415e050..00ed279176 100644
--- a/py/mkrules.mk
+++ b/py/mkrules.mk
@@ -71,7 +71,7 @@ $(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h
 
 $(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) | $(HEADER_BUILD)/mpversion.h
 	$(ECHO) "GEN $@"
-	$(Q)$(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $? >$(HEADER_BUILD)/qstr.i.last;
+	$(Q)$(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $(if $?,$?,$^) >$(HEADER_BUILD)/qstr.i.last;
 
 $(HEADER_BUILD)/qstr.split: $(HEADER_BUILD)/qstr.i.last
 	$(ECHO) "GEN $@"