Twitter posted worse-than-expected third quarter revenue and profit on Thursday, which the company attributed to advertising problems that included product bugs and unusually low demand over the summer, sending its shares down nearly 21 percent.

The company had forecast that third quarter revenue growth would lag the first two quarters, partly because of the ending of some older ad formats. But it also encountered unexpected problems, such as bugs which impacted its ability to target ads and share data with ad partners, and fewer big events compared with the previous summer.

However, the social media platform did record a rise in daily users who see ads on the site, beating analyst estimates.

Twitter’s revenue rose 9 percent from a year earlier to $824 million, missing Wall Street expectations of $874 million, according to data from Refinitiv. Total advertising revenue was $702 million, an increase of 8 percent year-over-year.