--- mkimage/imagetool-uncompressed.py
+++ mkimage/imagetool-uncompressed.py
@@ -46,7 +47,7 @@
 f = open(args.bootimage, "wb")
 
 for m in mem:
-   f.write(chr(m))
+   f.write(chr(m).encode('latin1'))
 
 f.write(kernel_image)
 f.close()