DTMF has duration of 79 but want minimum 80

I have had problems with DTMF tones not getting sent.
Seeing this on the CLI:
[2016-10-06 09:53:03] DTMF[21109][C-00004ffc]: channel.c:4192 __ast_read: DTMF end ‘4’ has duration 79 but want minimum 80, emulating on SIP/5349-000095e9

What does that mean?

The SIP trunk is to a Digium PRI gateway.