Market Regulation Services Inc. cancelled trading in more than 1 million CoolBrands International Inc. shares Wednesday after news that the frozen-treats maker had lost a major contract sparked a share selloff.

CoolBrands stock plunged 39% to $10.30 during the final 36 minutes of trading Wednesday on heavy volume. But all the trades that took place during that period were cancelled, leaving the final trade at $16.90 on the Toronto Stock Exchange, down $1.10 on the day.

The company, which makes ice-cream products under the Weight Watchers, Atkins and Eskimo Pie banners, said in a brief statement Wednesday that its licence agreement with Weight Watchers will cease on Sept. 28. Analysts estimate the contract will account for as much as 18% of CoolBrands’ sales this year.

In a report in the Globe and Mail, RS said the trades were cancelled because the company failed to alert RS that the news was coming, and failed to ask the regulator to halt trading in its shares.

The regulator cancelled every trade in the company’s stock after 3:24.

The company’s news release came out at 3:22 p.m. At 3:24, the first trade took place and the shares tumbled. At one point during the 36-minute period, the stock price dipped as low as $10.20.