Shares of Palo Alto Networks were down more than 12 percent in after-hours trading today, as the security vendor issued a forecast too conservative for investors' tastes.

Revenues for Palo Alto's first quarter, which ended Oct. 31, were up 34 percent compared with last year's, the company reported today. But for its second quarter, Palo Alto expects revenues of $426 million to $432 million. Analysts polled by Zacks were expecting $438.3 million.

Palo Alto's stock had fallen to $140.78 at press time.

Interest in Palo Alto's next-generation firewalls has continued to fuel growth. But for the last couple of quarters, investors have seemed wary that the growth might start to level off.

Palo Alto reported first-quarter revenues of $398.1 million, compared with $297.2 million in the same quarter a year ago.

The company's net losses were $68.1 million, or 69 cents per share, compared with losses of $39.9 million, or 47 cents per share, a year ago.

Non-GAAP net income of 55 cents beat the analysts' estimate of 53 cents, according to Thomson Financial.