diff --git a/scripts/imgtool/image.py b/scripts/imgtool/image.py
index 79a342d..efd20d7 100644
--- a/scripts/imgtool/image.py
+++ b/scripts/imgtool/image.py
@@ -125,7 +125,7 @@
             pubbytes = sha.digest()
             tlv.add('KEYHASH', pubbytes)
 
-            sig = key.sign(self.payload)
+            sig = key.sign(bytes(self.payload))
             tlv.add(key.sig_tlv(), sig)
 
         self.payload += tlv.get()