diff --git a/youtube_dl/FileDownloader.py b/youtube_dl/FileDownloader.py
index 47f678811..b0f26fc6f 100644
--- a/youtube_dl/FileDownloader.py
+++ b/youtube_dl/FileDownloader.py
@@ -557,7 +557,7 @@ class FileDownloader(object):
             # Progress message
             speed = self.calc_speed(start, time.time(), byte_counter - resume_len)
             if data_len is None:
-                self.report_progress('Unknown %', data_len_str, speed_str, 'Unknown ETA')
+                self.report_progress('Unknown %', data_len_str, speed, 'Unknown ETA')
                 eta = None
             else:
                 percent = self.calc_percent(byte_counter, data_len)