diff --git a/ports/stm32/Makefile b/ports/stm32/Makefile
index 9a66d3c10b..6c71f27573 100644
--- a/ports/stm32/Makefile
+++ b/ports/stm32/Makefile
@@ -18,8 +18,8 @@ include $(BOARD_DIR)/mpconfigboard.mk
 QSTR_GENERATED_HEADERS = $(BUILD)/pins_qstr.h $(BUILD)/modstm_qstr.h
 
 # qstr definitions (must come before including py.mk)
-QSTR_DEFS = qstrdefsport.h $(QSTR_GENERATED_HEADERS)
-QSTR_GLOBAL_DEPENDENCIES = mpconfigboard_common.h $(BOARD_DIR)/mpconfigboard.h $(QSTR_GENERATED_HEADERS)
+QSTR_DEFS += qstrdefsport.h $(QSTR_GENERATED_HEADERS)
+QSTR_GLOBAL_DEPENDENCIES += mpconfigboard_common.h $(BOARD_DIR)/mpconfigboard.h $(QSTR_GENERATED_HEADERS)
 
 # MicroPython feature configurations
 MICROPY_ROM_TEXT_COMPRESSION ?= 1