While operators remain optimistic, the performance index for restaurants edged slightly lower in December because of softer same-store sales.

According to the National Restaurant Association, December’s Restaurant Performance Index (RPI) was 100.5, down 0.6% from November and the first time the index has dropped in three months, according to a news release from the Washington, D.C.-based group.

Even with the decline, the index was above 100 for the tenth consecutive month, which the release said indicates operators think the restaurant sector is still expanding.

“The December decline in the RPI was due to a dip in the current situation indicators, which in turn was partly caused by inclement weather in large parts of the country,” Hudson Riehle, senior vice president of the research and knowledge group for the association, said in the release. “Despite the softer December results, restaurant operators remain generally optimistic about business conditions in the months ahead.”

The current situation index — accounting for one half of the value of the overall index — was pegged at 99.5 in December, down 1.7% from November and the lowest level in ten months.

Only 44% of restaurant operators reported stronger same-store sales in December, compared with 57% who reported gains in November.

The December results showed that 34% of operators reported customer growth between December 2012 and December 2013, down from 47% who reported a traffic gain in November compared with year-ago levels.

Despite the December setback, restaurant operators are still feeling good about the future, according to the release.

The expectations index stood at 101.5 in December, up 0.4% from November. The release said that December was the fourteenth consecutive month in which the index stood above 100.

Operators also expect sales to grow, with 38% of those polled indicating they expect higher sales in the next six month period compared to year-ago levels. Nearly half of all operators expect business to remain about the same, and 13% predicted lower sales compared with the same period a year ago.

The economic outlook for most operators was stable; 56% expect economic conditions to remain about unchanged, 28% said the economy will improve and 13% predicted a decline in U.S. economic prospects, according to the release.