diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index 1fd0403..1889aae 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -90,8 +90,33 @@
     uint8_t image_ok;
 };
 
-#define BOOT_STATUS_STATE_COUNT 3
-#define BOOT_STATUS_MAX_ENTRIES 128
+#if defined(__BOOTSIM__)
+#define BOOT_MAX_IMG_SECTORS       128
+#elif defined(__ZEPHYR__)
+#define BOOT_MAX_IMG_SECTORS       CONFIG_BOOT_MAX_IMG_SECTORS
+#elif defined(MCUBOOT_MYNEWT)
+#define BOOT_MAX_IMG_SECTORS       MYNEWT_VAL(BOOTUTIL_MAX_IMG_SECTORS)
+#else
+#error "Invalid target OS"
+#endif
+
+/*
+ * The current flashmap API does not check the amount of space allocated when
+ * loading sector data from the flash device, allowing for smaller counts here
+ * would most surely incur in overruns.
+ *
+ * TODO: make flashmap API receive the current sector array size.
+ */
+#if BOOT_MAX_IMG_SECTORS < 32
+#error "Too few sectors, please increase BOOT_MAX_IMG_SECTORS to at least 32"
+#endif
+
+/** Number of image slots in flash; currently limited to two. */
+#define BOOT_NUM_SLOTS             2
+
+/** Maximum number of image sectors supported by the bootloader. */
+#define BOOT_STATUS_STATE_COUNT    3
+#define BOOT_STATUS_MAX_ENTRIES    BOOT_MAX_IMG_SECTORS
 
 #define BOOT_STATUS_SOURCE_NONE    0
 #define BOOT_STATUS_SOURCE_SCRATCH 1
@@ -105,12 +130,6 @@
 
 extern const uint32_t BOOT_MAGIC_SZ;
 
-/** Number of image slots in flash; currently limited to two. */
-#define BOOT_NUM_SLOTS              2
-
-/** Maximum number of image sectors supported by the bootloader. */
-#define BOOT_MAX_IMG_SECTORS        120
-
 /**
  * Compatibility shim for flash sector type.
  *
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 82e17c5..e2762bd 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -337,14 +337,22 @@
     size_t size_0, size_1;
     size_t i;
 
-    /* Ensure both image slots have identical sector layouts. */
-    if (num_sectors_0 != num_sectors_1) {
+    if (num_sectors_0 > BOOT_MAX_IMG_SECTORS || num_sectors_1 > BOOT_MAX_IMG_SECTORS) {
+        BOOT_LOG_WRN("Cannot upgrade: more sectors than allowed");
         return 0;
     }
+
+    /* Ensure both image slots have identical sector layouts. */
+    if (num_sectors_0 != num_sectors_1) {
+        BOOT_LOG_WRN("Cannot upgrade: number of sectors differ between slots");
+        return 0;
+    }
+
     for (i = 0; i < num_sectors_0; i++) {
         size_0 = boot_img_sector_size(&boot_data, 0, i);
         size_1 = boot_img_sector_size(&boot_data, 1, i);
         if (size_0 != size_1) {
+            BOOT_LOG_WRN("Cannot upgrade: an incompatible sector was found");
             return 0;
         }
     }
@@ -1336,6 +1344,8 @@
     /* Determine the sector layout of the image slots and scratch area. */
     rc = boot_read_sectors();
     if (rc != 0) {
+        BOOT_LOG_WRN("Failed reading sectors; BOOT_MAX_IMG_SECTORS=%d - too small?",
+                BOOT_MAX_IMG_SECTORS);
         goto out;
     }