diff --git a/boot/zephyr/flash_map.c b/boot/zephyr/flash_map.c
index 431eacf..edb02ed 100644
--- a/boot/zephyr/flash_map.c
+++ b/boot/zephyr/flash_map.c
@@ -125,6 +125,11 @@
 	return slot + FLASH_AREA_IMAGE_0;
 }
 
+#ifndef FLASH_AREA_IMAGE_SECTOR_SIZE
+#warning "Missing FLASH_AREA_IMAGE_SECTOR_SIZE; assuming scratch size instead"
+#define FLASH_AREA_IMAGE_SECTOR_SIZE FLASH_AREA_IMAGE_SCRATCH_SIZE
+#endif
+
 /*
  * Lookup the sector map for a given flash area.  This should fill in
  * `ret` with all of the sectors in the area.  `*cnt` will be set to
@@ -136,8 +141,8 @@
 	uint32_t off;
 	uint32_t len;
 	uint32_t max_cnt = *cnt;
+	uint32_t rem_len;
 
-	BOOT_LOG_DBG("lookup area %d", idx);
 	/*
 	 * This simple layout has uniform slots, so just fill in the
 	 * right one.
@@ -166,15 +171,30 @@
 		return -1;
 	}
 
+	BOOT_LOG_DBG("area %d: offset=0x%x, length=0x%x, sector size=0x%x",
+		     idx, off, len, FLASH_AREA_IMAGE_SECTOR_SIZE);
+
+	rem_len = len;
 	*cnt = 0;
-	while (len > 0 && *cnt < max_cnt) {
+	while (rem_len > 0 && *cnt < max_cnt) {
+		if (rem_len < FLASH_AREA_IMAGE_SECTOR_SIZE) {
+			BOOT_LOG_ERR("area %d size 0x%x not divisible by sector size 0x%x",
+				     idx, len, FLASH_AREA_IMAGE_SECTOR_SIZE);
+			return -1;
+		}
+
 		ret[*cnt].fa_id = idx;
 		ret[*cnt].fa_device_id = 0;
 		ret[*cnt].pad16 = 0;
-		ret[*cnt].fa_off = off + (FLASH_AREA_IMAGE_SCRATCH_SIZE * (*cnt));
-		ret[*cnt].fa_size = FLASH_AREA_IMAGE_SCRATCH_SIZE;
+		ret[*cnt].fa_off = off + (FLASH_AREA_IMAGE_SECTOR_SIZE * (*cnt));
+		ret[*cnt].fa_size = FLASH_AREA_IMAGE_SECTOR_SIZE;
 		*cnt = *cnt + 1;
-		len -= FLASH_AREA_IMAGE_SCRATCH_SIZE;
+		rem_len -= FLASH_AREA_IMAGE_SECTOR_SIZE;
+	}
+
+	if (*cnt >= max_cnt) {
+		BOOT_LOG_ERR("flash area %d sector count overflow", idx);
+		return -1;
 	}
 
 	return 0;