The Nigerian National Petroleum Corporation (NNPC) “failed to pay” about $16 billion in revenue owed to the state’s treasury, according to an audit.

Samuel Ukura, the auditor-general, issued a report Monday to the national assembly to disclose details about the missing funds. But the NNPC on Wednesday said in a statement the amount was closer to $1.64 billion. It said the audit failed to account for maintenance fees, a subsidy on oil products, and losses from crude-oil theft.

NNPC, which is state-owned, is obligated by law to transfer its oil revenues to the government, which then decides how much is returned to the company. But an audit of the 2014 figures found a shortfall of $16 billion.

Although corruption at NNPC has been blamed for missing funds in the past, revenue loss does indeed begin at the pipes. Poor infrastructure has resulted in people siphoning crude oil from decades-old, rusted pipes into drums. These drums are then transported on boats to illicit refineries, from where oil is sold on the black market.

“It’s impossible to estimate how much oil has been lost in this manner,” Kolawole Banwo, a senior program officer at Nigeria’s Civil Society Legislative Advocacy Centre, told the BBC last year. “That’s because there is no independent metering facility to measure the quantity of oil that is extracted.”