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();
 }