diff --git a/scripts/imgtool/image.py b/scripts/imgtool/image.py
index 3ccd86f..4b7125e 100644
--- a/scripts/imgtool/image.py
+++ b/scripts/imgtool/image.py
@@ -231,7 +231,7 @@
             ) # }
         assert struct.calcsize(fmt) == IMAGE_HEADER_SIZE
         header = struct.pack(fmt,
-                IMAGE_MAGIC,
+                self._image_magic(),
                 0, # LoadAddr
                 self.header_size,
                 0, # Pad1
@@ -245,6 +245,9 @@
         self.payload = bytearray(self.payload)
         self.payload[:len(header)] = header
 
+    def _image_magic(self):
+        return IMAGE_MAGIC
+
     def _trailer_size(self, write_size, max_sectors, overwrite_only):
         # NOTE: should already be checked by the argument parser
         if overwrite_only: