Argentina went from robust to bust in a matter of months, leaving investors scratching their heads. Among the mysteries: why the country’s record hoard of currency reserves—often seen as shield against foreign-exchange volatility—did so little to stem the crisis.

The lesson seems to be that loads of dollar reserves alone can’t make up for weakness in a country’s economic underpinnings. Foreign investors who had been burned by Argentina’s repeated defaults over the years, reassured by the country’s rising reserve bulwark, edged back into the country in recent years, buying up assets like a 100-year government bond issued in June last year.

“If they thought they could defend the currency by selling the reserves, to be honest they wouldn’t have raised interest rates to 40%,” said Carl Shepherd, portfolio manager at Newton Investment Management. When a currency is falling, central banks can sell dollar reserves and buy the local currency to prop it up. But in Argentina’s case, the maneuver failed to reassure investors.

Argentina’s situation may also shed light on whether reserves will prove more useful against a rising dollar elsewhere among the world’s most vulnerable economies. Turkey, whose currency has weakened sharply against the dollar this year, is a clear focus for investors.

When local currencies weaken as the dollar strengthens, this can stoke inflation by making imported goods more expensive. Countries that issue debt in dollars but raise revenue in a local currency may also struggle to pay back their debts, something holding dollar reserves should help to avoid.