diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index e7b9c1b..7b6ae63 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -110,7 +110,7 @@
 }
 
 #define ERASED_VAL 0xff
-uint8_t flash_area_erased_val(const struct flash_area *fap)
+__weak uint8_t flash_area_erased_val(const struct flash_area *fap)
 {
     (void)fap;
     return ERASED_VAL;