Berlin’s brothels this week hosted unlikely visitors: financial employees seeking a cut of the prostitutes’ earnings, the Times of London reported.

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Seeking to boost revenue for the German capital, tax inspectors with clipboards and stopwatches were sent to study financial turnover at the houses of ill repute, where prostitution has been legal since 2002. That same year, prostitutes were added to the pool of taxpayers, yet few have been filing.

“Prostitution is a strong economic sector in the capital,” said Gerry Woop, spokesman for the economic administration in Berlin, home to 7,000 sex workers.

Berlin’s prostitutes bring in a yearly turnover of about $402 million, making the sex industry more lucrative than domestic building, painting and decorating, according to the German Institute for Economic Research.

Under the new rules, prostitutes must give $40.50 a day to the brothel owner as an advance payment on the tax.