Southern California’s housing market weakened in May, with sales and prices dropping in all of the region’s counties, providing further evidence of a weak spring shopping season.

Sales fell 17.4% from the same month a year ago, with a total of 18,394 newly built and previously owned homes sold in the region last month, according to San Diego real estate research firm DataQuick. That sales tally was a three-year low for the month. The drop was particularly pronounced because a popular tax credit for buyers that had been fueling sales expired during the same period last year.

The region’s median home price fell 8.2% from the same month a year earlier to $280,000. It was the largest amount the median has fallen in 20 months. Lingering economic uncertainties have left the market soft ever since the expiration of the tax credits.