diff --git a/util/uf2conv.py b/util/uf2conv.py
index 7f5645414a..eb289b5039 100755
--- a/util/uf2conv.py
+++ b/util/uf2conv.py
@@ -73,7 +73,7 @@ def convert_from_uf2(buf):
             assert False, "Non-word padding size at " + ptr
         while padding > 0:
             padding -= 4
-            outp += b"\x00\x00\x00\x00"
+            outp.append(b"\x00\x00\x00\x00")
         if familyid == 0x0 or ((hd[2] & 0x2000) and familyid == hd[7]):
             outp.append(block[32 : 32 + datalen])
         curraddr = newaddr + datalen