It seems underperforming CEOs have a new life preserver: Brexit. Britain’s vote to leave the EU in June certainly caught companies off guard. Six months on, has Brexit begun to take a toll on profits, or is it just an excuse? GM offers an answer.

GM announced its earnings earlier this week and said Britain’s vote to leave the EU hit it’s European results. The company recorded an adjusted loss before interest and tax of $300 million in the region, mainly because of the drop in the British pound against the dollar. CFO Chuck Stevens said: “Absent Brexit we would have broken even in Europe.”

Well, not quite. It didn’t take long for a brokerage analyst (thank goodness there’s still one or two around) to point out that GM‘s European business would have been in the red even without the currency dip.

Of course, shareholders might also ask why GM didn’t hedge its currency risk. It’s certainly sophisticated enough to manage its currency exposure.

It’s easy to see why GM latched on to Brexit to explain a missed target. Brexit is creating a great deal of uncertainty about European trade, regulation and taxes, as well as the region’s future economic growth. And even though the precise contours of Brexit won’t be known for years, it is already becoming a factor in companies’ strategic planning.

But whether it is hitting a company’s bottom line today is a matter of debate. Or, in the case of GM, it is a matter of looking closely at the numbers. And they showed Brexit wasn’t to blame.

For companies, the lesson is clear. Blaming Brexit might be a tempting impulse, but it is sure to dent a CFO’s credibility if the data do not support it.

It’s a trap that companies (and quite a few analysts, to be honest) fall into occasionally. They point to popular trends or events to explain their financial results when those factors didn’t really have much effect on the business. Unusually hot or cold weather, for example, is a favorite.

There’s little doubt that Brexit is making life harder for companies with significant operations in Europe. But they shouldn’t use it to distract attention from their underlying business performance.