Slack failing at SSL?

weechat-output.txt

16:24:12    team  -- | gnutls: connected using 512-bit Diffie-Hellman shared secret exchange
16:24:12    team  -- | gnutls: receiving 2 certificates
16:24:12    team  -- |  - certificate[1] info:
16:24:12    team  -- |    - subject `C=US,ST=California,L=San Francisco,O=Tiny Speck,CN=*.irc.slack.com', issuer `C=US,O=GeoTrust Inc.,CN=GeoTrust SSL CA - G3', RSA key 2048
                     | bits, signed using RSA-SHA256, activated `2015-11-25 00:00:00 UTC', expires `2017-12-02 23:59:59 UTC', SHA-1 fingerprint
                     | `ed4dcb04c89dc80d43e65437e7c4632dcf50ac86'
16:24:12    team  -- |  - certificate[2] info:
16:24:12    team  -- |    - subject `C=US,O=GeoTrust Inc.,CN=GeoTrust SSL CA - G3', issuer `C=US,O=GeoTrust Inc.,CN=GeoTrust Global CA', RSA key 2048 bits, signed using
                     | RSA-SHA256, activated `2013-11-05 21:36:50 UTC', expires `2022-05-20 21:36:50 UTC', SHA-1 fingerprint `5aeaee3f7f2a9449cebafeec68fdd184f20124a7'
16:24:12    team =!= | gnutls: peer's certificate is NOT trusted
16:24:12    team =!= | gnutls: peer's certificate issuer is unknown
16:24:12    team =!= | irc: TLS handshake failed
16:24:12    team =!= | irc: error: Error in the certificate.