I'm very curious what poll you're referring to there that has him after "don't know".
I used to like him. I wish he was all that you call him.
What did I 'call him' that you disagree with?
Unrelated: while YouGov is indeed online, it chooses which people to send the surveys to and has long-term profiles on each of them, so unlike a silly Twitter poll it has decent statistical relevance and the biases seem to be persistent, well quantified, and generally accounted for — you shouldn’t dismiss it just because you don’t like what it’s saying.
Now, I support Sanders and quite like Corbyn minus the Brexit and the antisemitism issues, but they both face a similar challenge: their core support bases aren't actually ideologically coherent, let alone aligned entirely behind the candidates' programs. Lots of leftists basically treat Sanders or Corbyn as beloved old conservative uncles, while insisting that nobody with Sanders' or Corbyn's actual (social democratic to democratic socialist) politics should encouraged within Left politics.