Nikkei 225 Stock Average futures tumbled, reversing an earlier gain, after the Bank of Japan stood pat on monetary policy when most economists were expecting further stimulus.

Contracts on the measure traded in Singapore sank 3.4 percent as of 12:07 p.m. in Tokyo after rising as much as 1.8 percent before the central bank’s announcement. The BOJ kept its bond-buying program at 80 trillion yen ($733 billion) a year and made no changes to its negative-interest rate or its program for purchasing exchange-traded funds.

“It’s a total shock,” said Nader Naeimi, the Sydney-based head of dynamic markets at AMP Capital Investors Ltd., which oversees about $120 billion. “From currencies to equities to everything -- you can see the reaction in the markets. I can’t believe this. It’s very disappointing.”

A slight majority of economists surveyed by Bloomberg last week, 23 out of 41, said they expected the BOJ to add to stimulus. Nineteen predicted the central bank would increase purchases of ETFs, eight expected a boost in bond buying and eight projected the BOJ would further cut its negative rate, the survey conducted April 15-21 showed.

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