United Airlines Holdings Inc. is sweetening the yield on a $2.25 billion junk-bond sale after investors expressed concerns about the aging fleet pledged as collateral, according to people familiar with the matter.

The airline is offering a yield of 11%, which was increased from initial discussions in the low 9% range for the deal, said the people, who asked not to be identified discussing a private transaction. The changes come after the three and five-year bonds had only received about $1.5 billion of orders as of Thursday morning, the people added.

Investors have so far proven eager to lend to companies that have been hit hard by the coronavirus pandemic. But the pushback on United’s deal suggests they still have their limits, and may be reluctant to jump in without the double-digit yields and iron-clad collateral seen in previous offerings.

The company has also added a clause that would trigger repayment of the bonds at a substantial premium to par, known as a make-whole, if the company files for bankruptcy, the people said.

Shares of United pared gains after Bloomberg reported on weak demand for the offering and later turned negative, trading as low as $22.45 from around $23.95 earlier. The S&P 500 Airlines Index, meanwhile, was up 1.4% at 3:46 p.m. in New York. The company’s shares had already taken a beating earlier this week after Warren Buffett dumped his stakes in the four U.S. biggest airline carriers, saying the industry’s prospects had been upended by the outbreak.

JPMorgan Chase & Co, which is leading the deal, declined to comment. A representative for United also declined to comment.

The notes are secured on a first priority basis by a pool of 360 aircraft owned by United. Some investors are concerned that these are not valuable enough to balance out the risk of investing in an airline whose business has been hit as governments across the globe have halted travel to help stem the spread of Covid-19.

“This is a good example of a firm being creative in financing unencumbered assets,” said John McClain, a money manager at Diamond Hill Capital Management. These are older planes that investors should be skeptical about financing, he added.

The aircraft are close to retirement with a weighted average age of 19 years, CreditSights analyst Roger King wrote in a note published Wednesday. That means many of the planes will not be flying when the five-year bond matures, he wrote.

United is taking advantage of a reopening junk-bond market to borrow, with the intention of using proceeds from the sale to pay down a $2 billion one-year loan it arranged with banks in March, a portion of which was bought by Apollo Global Management Inc. The lenders had a first priority claim on planes whose value exceeded the amount of the loan, according to a filing.

United raised about $4 billion of new liquidity through three new term loans, new aircraft financings, and an equity offering through April 29, bringing total liquidity to about $9.6 billion, according to its first quarter earnings release.

The company has used other collateral on bank loans to shore up cash. A $500 million loan was secured against spare parts, while a $250 million deal was secured by liens on spare engines.

This article was provided by Bloomberg News.