Image source: Rubicon Project.

What: Shares of The Rubicon Project (NYSE:RUBI) are getting crushed today, down by 14% as of 12:22 p.m. ET, after the company reported first-quarter earnings.

So what: Non-GAAP revenue in the first quarter came in at $63.6 million, which translated into non-GAAP earnings per share of $0.31. Adjusted EBITDA was $15.5 million. Both top- and bottom-line results topped analyst expectations, but the company's lackluster guidance is really what's driving the sell-off.

Now what: The company expects second-quarter adjusted sales in the range of $61 million to $64 million, with adjusted EBITDA of $9 million to $11 million. Non-GAAP earnings per share in the current quarter should be $0.08 to $0.10. For the full year, adjusted sales should be $275 million to $295 million, with adjusted earnings of $0.75 to $0.85 per share. The cautious outlook didn't inspire a lot of confidence with investors, as management did not want to raise expectations despite the strong first-quarter results.