Hedge funds’ annual letters tend to be an opportunity for fund managers to pat themselves on the back for investments well-managed, congratulate investors for being smart enough to entrust them with their money, and raise expectations for the returns they plan to reap in the year ahead.

But activist investor Bill Ackman, usually not one to shy away from giving himself a round of applause, instead humbled himself in his year-end letter from his firm Pershing Square Capital. It’s hard to do much but hang one’s head after losing 20.5 percent in a relatively flat market—Pershing’s worst-ever performance. But it’s safe to say no investor was holding his or her breath for a section titled “humility” at the end of Ackman’s annual roundup.

“I have often stated that in order to be a great investor one needs to first have the confidence to invest without perfect information at a time when others are highly skeptical about the opportunity you are pursuing. This confidence, however, has to be carefully balanced by the humility to recognize when you are wrong,” he wrote. “While no one here is enthusiastic about delivering our worst performance year in history in 2015, it certainly does a good job reinforcing the humility-side of the equation that is necessary for long-term investment performance.”

Ackman practiced what he preached in terms of the humility side, opening the letter with an admission that the faults of his firm are his own. There was no real financial crisis except in energy and commodities, he wrote. There was no great war or global economic slowdown in 2015. “Yet, the Pershing Square funds suffered their greatest peak-to-trough decline and worst annual performance ever.”

The rest of Ackman’s note cycles through the missteps his fund made, particularly in Valeant, Pershing Square’s biggest bet of the year, which became the bust of the summer amid a debate over drug pricing that slashed its stock price. The billionaire investor now admits not selling some of its stake at its high was a bad decision.

“When the stock price rose this summer to the mid-$200s per share, we did not sell as we believed it was probable the company would likely complete additional transactions that would meaningfully increase intrinsic value,” he wrote. “In retrospect, this was a very costly mistake.” He said the fund made a similar mistake with Canadian Pacific, Canada's second biggest railway.

Ackman also announced that Paul Hilal, his number two at Pershing Square, former Harvard roommate, and groomsman at his wedding, is leaving the firm.

“We knew that it can be difficult for close friends to work for and even partner with the other. We both thought our arrangement could last three to five years and possibly more if things worked out,” Ackman wrote on his exit. “This January marks a full decade of Paul’s commitment to the firm.”

Ackman did his best to nurse the wounds of 2015, though his own letter mostly poured salt water over them. “Fortunately,” he resolved, “the lessons we have learned in 2015 should be easy to avoid in the future.” As of last week, Pershing Square was down another 14.5 percent so far in 2016.