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Daily voucher company Groupon has reported an unexpected loss.

In its first set of results since listing on Nasdaq stock exchange in November, Groupon reported a net loss of $42.7m (£27.0m), when a small profit had been expected.

The loss for the last three months of 2011 compares with a loss of $378.6m in the same period of 2010.

Shares in the company fell 13% in after-hours trading to $21.35, still above the listing price of $20.

The number of people who bought a Groupon in the fourth quarter rose to 33 million, which was up 20% from the third quarter, but still below expectations.

"The number of active customers came in short. That means not enough people are buying groupons," said Sameet Sinha at B. Riley and Company.

"Yes, you can get fewer people to buy more, but how long can that continue? You need to start investing in new customer growth."

The net loss for the whole of 2011 was $350.8m, down from $456.3m in 2010.

Groupon blamed the fourth quarter loss on $34.8m of tax expenses in some of its international businesses, where it said it had paid an effective tax rate of approximately 1,600%.

It also said that it had made extra provisions for income tax after establishing its international headquarters in Switzerland.