Instant death when attacking Baron with Braum

I was just playing a normal Game with Braum as Support. I was bound with Kalista but I don't know if that matters. In lategame when going for Baron we went into the Baron pit and started attacking. As soon as Baron Nashor hit me once, I was executed on instant. I had 4000HP, round about 250 armor and 150 Magicresist. Happened twice that game.(every time we went for Baron) My item build was as following: (with homeguard), , , , , ,