diff --git a/test/rsrc/convert_stub.py b/test/rsrc/convert_stub.py
index 0f2dadd92..aa0d2ea72 100755
--- a/test/rsrc/convert_stub.py
+++ b/test/rsrc/convert_stub.py
@@ -10,6 +10,10 @@ import sys
 def convert(in_file, out_file, tag):
     """Copy `in_file` to `out_file` and append the string `tag`.
     """
+    # On Python 3, encode the tag argument as bytes.
+    if not isinstance(tag, bytes):
+        tag = tag.encode('utf8')
+
     with open(out_file, 'wb') as out_f:
         with open(in_file, 'rb') as in_f:
             out_f.write(in_f.read())