Slack, the popular cloud-based workplace tool, has been out looking to raise new funding in recent weeks at what sources said is a striking valuation of close to $5 billion.

The Wall Street Journal reported Wednesday that the startup is seeking to raise about $150 million in new investment, although it might raise even more (Bloomberg said the round could be as high as $300 million).

But other valuations of cloud software startups have lost of a lot of their altitude in the last few months, and that could mean privately-backed companies are not as valuable as they were as recently as last fall.

Maybe Slack will be the exception. According to sources, its potentially lofty valuation — amid a shrinking funding environment — is based on revenue that has grown 10 times year over year. Slack has said that its annual run rate — its projected revenue for the next 12 months — is more than $60 million.

That valuation is much higher than the $2.8 billion Slack received when it raised $160 million just 11 months ago. In total, it has raised about $340 million in funding.

Things in the cloud space have been rocky since then. Just look at the Bessemer Venture Partners Cloud Index, which tracks the market values of 44 publicly traded cloud software outfits including Salesforce.com, Workday, Box and Marketo, among others. Shares of those companies have only just now started to bounce back, after declining by a combined $66 billion over two months.

That’s why Slack — the darling of the space right now — may have a harder time getting the valuation it could be seeking. “It’s a good business, and they’re doing well, but in this market, they’ll get maybe $4 billion,” said one person familiar with Slack’s process.

That’s still impressive, and it could be much worse. After all, last year’s unicorn is this year’s down-round candidate. In the last three months of 2015, more than a quarter of the companies raising a Series E round or later closed at a lower valuation than their prior rounds, according to a report from Fenwick and West, the Silicon Valley law firm.

Still, there’s no question Slack is on a roll. As of December, it disclosed it had more than two million daily users, four times more than a year ago, and more than 570,000 of those were at organizations paying for their subscriptions. It’s taking over office communications at small- and mid-sized companies, but it still needs to break into bigger companies.

Slack set up an $80 million investment fund to boost startups building Slack enhancements, and just today it surprised everyone by rolling out a beta test of voice and video calling matching features that Cisco Systems added to its Spark product in December.

One other factor that might make Slack a good bet for investors is continued interest by bigger players in the fast-growing company for a possible acquisition. Slack management has ignored overtures — including from Microsoft — but this certainly makes it more enticing.

A Slack spokesperson declined to comment on any funding.