If any investor wanted to get a sense of how tough the Nigerian banking sector can be, they could do worse than speak to the world’s third-biggest buyout firm.

Bloomberg reports that Carlyle Group LP made its first investment in Africa’s main oil producer in August 2014, buying $147 million of shares in Lagos-based Diamond Bank Plc. At the time, it said the lender was well-positioned to benefit from Nigeria’s status as one of the fastest-growing economies on the continent and enthused how it could become one of the largest financial institutions in West Africa.

Since then, the bank’s shares have lost 94 percent of their value in dollars, and Carlyle is yet to make another Nigerian purchase.

According to Bloomberg, the trigger for the collapse was the 2014 oil-price crash, which started just as Carlyle made its investment. That hammered Nigeria’s economy — growth was 0.8 percent last year, having averaged more than 6 percent in the first half of the decade — and caused non-performing loans to soar. Lenders also had to contend with a shrinking economy in 2016.

Diamond Bank had a dire November, with its stock dropping 54 percent amid downgrades by Moody’s Investors Service and S&P Global Ratings, its worst monthly performance yet. Some traders also fretted about whether it would have enough cash to repay a $200 million Eurobond maturing in May, which the bank’s management has said won’t be a problem.

The shares rose more than 9 percent on Monday, its first gain in six days, after Diamond Bank announced late last week it’s close to selling its U.K. arm and the Nigerian central bank will allow it to operate as a national, rather than international, firm. That would free up some capital, but will be of little consolation to Carlyle, which was looking for regional growth.

Diamond Bank’s scarcely the only Nigerian lender to suffer in the past four years. Even bigger rivals, including Guaranty Trust Bank Plc and Zenith Bank Plc, have seen their share prices fall by more than 45 percent in dollar terms. The stock of Bob Diamond’s Atlas Mara Ltd., the main holding of which is Union Bank Nigeria Plc, is trading near a record low and down 81 percent since it was listed five years ago.

With tense presidential elections scheduled for February and the naira coming under pressure amid a fresh slump in oil prices, investors don’t seem to think there will be an improvement soon. Guaranty Trust Bank and Zenith both have forward price-to-earnings ratios of less than 5.5, which compares with that of 8.1 for MSCI’s emerging-markets banking index