diff --git a/youtube_dl/downloader/ism.py b/youtube_dl/downloader/ism.py
index 9b001ecff4..138564267a 100644
--- a/youtube_dl/downloader/ism.py
+++ b/youtube_dl/downloader/ism.py
@@ -139,7 +139,7 @@ def write_piff_header(stream, params):
         sample_entry_payload += u16.pack(0x18)  # depth
         sample_entry_payload += s16.pack(-1)  # pre defined
 
-        codec_private_data = binascii.unhexlify(params['codec_private_data'])
+        codec_private_data = binascii.unhexlify(params['codec_private_data'].encode('utf-8'))
         if fourcc in ('H264', 'AVC1'):
             sps, pps = codec_private_data.split(u32.pack(1))[1:]
             avcc_payload = u8.pack(1)  # configuration version