At least two oil tanker owners are staying away from Nigeria after several companies received backdated tax bills totaling millions of dollars.

Multiple businesses received demands from Nigeria’s Federal Inland Revenue Service, according to a member notice by industry group Intertanko seen by Bloomberg. They cover the period from 2010 to 2019 and range in amount from $400,000 to $1.1 million per vessel. In aggregate, some claims reach tens of millions of dollars.

As a result, at least two shipowners, who asked not to be identified discussing commercial matters, are steering clear of Nigerian ports to avoid the risk of having their ships arrested. Tanker earnings from West Africa to Europe jumped 27% on Tuesday, according to Baltic Exchange data, the biggest daily increase in a month.

Ships staying away from Nigeria makes it easier for owners willing to travel there to command higher premiums for their vessels.

Many of the tax bills referred to a previous law published by Nigeria’s revenue service in July 2021. That measure says any vessel carrying crude oil, gas or refined fuels from Nigeria is liable to pay tax there.

Intertanko couldn’t immediately comment. Nigeria’s FIRS didn’t respond to multiple requests for comment.