diff --git a/boot/mbed/include/flash_map_backend/flash_map_backend.h b/boot/mbed/include/flash_map_backend/flash_map_backend.h
index f2bad16..34bac73 100644
--- a/boot/mbed/include/flash_map_backend/flash_map_backend.h
+++ b/boot/mbed/include/flash_map_backend/flash_map_backend.h
@@ -143,7 +143,7 @@
 /*
  * Alignment restriction for flash writes.
  */
-uint8_t flash_area_align(const struct flash_area * fap);
+uint32_t flash_area_align(const struct flash_area * fap);
 
 /*
  * What is value is read from erased flash bytes.
diff --git a/boot/mbed/src/flash_map_backend.cpp b/boot/mbed/src/flash_map_backend.cpp
index 28afec8..8f5137d 100644
--- a/boot/mbed/src/flash_map_backend.cpp
+++ b/boot/mbed/src/flash_map_backend.cpp
@@ -191,7 +191,7 @@
     return bd->erase(off, len);
 }
 
-uint8_t flash_area_align(const struct flash_area* fap) {
+uint32_t flash_area_align(const struct flash_area* fap) {
     mbed::BlockDevice* bd = flash_map_bd[fap->fa_id];
     return bd->get_program_size();
 }