diff --git a/scripts/imgtool/image.py b/scripts/imgtool/image.py
index 9bb4801..251aa9a 100644
--- a/scripts/imgtool/image.py
+++ b/scripts/imgtool/image.py
@@ -202,7 +202,7 @@
             for i in range(dependencies_num):
                 e = STRUCT_ENDIAN_DICT[self.endian]
                 payload = struct.pack(
-                                e + 'I'+'BBHI',
+                                e + 'B3x'+'BBHI',
                                 int(dependencies[DEP_IMAGES_KEY][i]),
                                 dependencies[DEP_VERSIONS_KEY][i].major,
                                 dependencies[DEP_VERSIONS_KEY][i].minor,