WASHINGTON (AP) — Americans pulled back substantially on signing contracts to buy homes in December, a possible sign of mounting affordability challenges as prices are rising faster than incomes.

The National Association of Realtors said Wednesday that its pending home sales index, which measures the numbers of purchase contracts signed, plunged 4.9% last month to 103.2. Still, lower mortgage rates mean that contract signings were up 4.6% from December 2018.

The average rate on a 30-year mortgage was 3.65% last week, down from 4.45% a year ago, according to mortgage buyer Freddie Mac. Cheaper borrowing costs helped boost demand for housing last year, but it also led to higher prices by the end of 2019 that could weigh on affordability going forward.

Contract signings fell last month in the Northeast, Midwest, South and West.