diff --git a/tools/targets.py b/tools/targets.py
index 84ae222b1c..edcd33181d 100644
--- a/tools/targets.py
+++ b/tools/targets.py
@@ -489,7 +489,7 @@ class MCU_NRF51Code(object):
             binh.merge(blh)
 
         with open(binf.replace(".bin", ".hex"), "w") as fileout:
-            binh.tofile(fileout, format='hex')
+            binh.write_hex_file(fileout, write_start_addr=False)
 
 class NCS36510TargetCode:
     @staticmethod