diff --git a/src/calibre/ebooks/rtf/output.py b/src/calibre/ebooks/rtf/output.py
index 0fc9b3cbb0..5738b7e6f4 100644
--- a/src/calibre/ebooks/rtf/output.py
+++ b/src/calibre/ebooks/rtf/output.py
@@ -30,7 +30,7 @@ def convert(self, oeb_book, output_path, input_plugin, opts, log):
 
         out_stream.seek(0)
         out_stream.truncate()
-        out_stream.write(content.encode(opts.output_encoding, 'replace'))
+        out_stream.write(content.encode('ascii', 'replace'))
 
         if close:
             out_stream.close()