Faulty mergers and acquisitions have spurred an uptick in insurance claims as dealmakers overstated profits or breached accounting rules, insurer American International Group Inc. said in a study of about 1,600 transactions.

“The bigger and more complicated a deal is, the more likely there is an unknown liability lingering,” Mary Duffy, AIG’s global head of M&A insurance, said in a statement Thursday. AIG published a review of more than $400 billion worth of transactions in the five years through 2015.

AIG and rivals including Warren Buffett’s Berkshire Hathaway Inc. and Allianz SE have been expanding efforts to insure risks tied to transactions, a market that’s reached records in the U.S. in recent years. Insurance broker Marsh Inc. said in a study last year that more than $11 billion in coverage was written by the end of 2015 as acquirers seek protection against higher-than-expected costs or setbacks including regulatory hurdles.

“The frequency of M&A insurance claims is rising as large deals prove risky,” AIG said in its report. The insurer said its average payout on the most severe mishaps is more than $20 million.

A fourth of all policies written on deals topping $1 billion led to a claim, AIG said. Worldwide, 134 M&A deals valued at $1 billion or more, that are pending or have been completed, were announced this year, compared with 111 for the same period in 2016, according to data compiled by Bloomberg.

Errors in financial data, such as undisclosed liabilities, misstatement of inventories and overstatement of profits, were the most common cause of claims, AIG’s study showed. Legal compliance issues came in at No. 2, while tax-related breaches and intellectual-property questions were also listed.

“We are paying sizable claims, sometimes writing eight-figure checks in different geographies,” Duffy said. “You can pick up all sorts of complex issues that are not flagged during the diligence process.”