Nutrisystem Inc. investors have sued the Pennsylvania-based diet and weight-loss company over its planned $1.3 billion sale to Tivity Health Inc., claiming that Nutrisystem brass has withheld key data ahead of a stockholder vote expected in the coming months.

In an 18-page complaint, shareholders Thursday questioned analyses by Nutrisystem’s Evercore Group financial advisers and said that the cash-and-stock deal, announced in December, had undervalued the company. Though a date has not yet been set for a shareholder vote, Nutrisystem and Nashville, Tennessee, area-based Tivity have said they plan to close the merger in the first quarter of 2019.