Stocks rose to records in October. One corner of the debt market had a rougher time.

Some securities in the $680 billion market for collateralized loan obligations, or CLOs, lost about 5% in October, reflecting worries about rising risk in the complex investment vehicles. The declines were a rare stumble for the CLO market, which has grown by about $350 billion in the past three years, according to data from S&P Global Market Intelligence, fueled by demand from government pensions, hedge funds and other yield-hungry investors.

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