diff --git a/sim/mcuboot-sys/csupport/flash_map.h b/sim/mcuboot-sys/csupport/flash_map.h
index 506f266..7622dd0 100644
--- a/sim/mcuboot-sys/csupport/flash_map.h
+++ b/sim/mcuboot-sys/csupport/flash_map.h
@@ -126,6 +126,11 @@
 uint8_t flash_area_align(const struct flash_area *);
 
 /*
+ * What is value is read from erased flash bytes.
+ */
+uint8_t flash_area_erased_val(const struct flash_area *);
+
+/*
  * Given flash area ID, return info about sectors within the area.
  */
 int flash_area_get_sectors(int fa_id, uint32_t *count,
diff --git a/sim/mcuboot-sys/csupport/run.c b/sim/mcuboot-sys/csupport/run.c
index ece8555..8238b39 100644
--- a/sim/mcuboot-sys/csupport/run.c
+++ b/sim/mcuboot-sys/csupport/run.c
@@ -52,6 +52,13 @@
     return sim_flash_align;
 }
 
+uint8_t sim_flash_erased_val = 0xff;
+uint8_t flash_area_erased_val(const struct flash_area *area)
+{
+    (void)area;
+    return sim_flash_erased_val;
+}
+
 struct area {
     struct flash_area whole;
     struct flash_area *areas;