Retirement planning is full of unknowns, from how long you’ll live to how much you’ll spend. But according to Grant Cardone, there’s one threat that could quietly sabotage your future: taxes.

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Here’s why the private equity fund manager and real estate investor believes that taxes can easily derail your retirement plans.

Many of us plan for retirement under the assumption that tax rates will be close to what they are right now, but Cardone — who recently launched the 10X Wealth Plan, a personalized financial coaching program — said this is an assumption we should not be making.

“Everybody assumes their retirement account’s going to handle them, but they don’t know what their tax [burden will be] when they’re 73 years old,” he told GOBankingRates. “In the year 2061, you have no clue what the taxes will be. Taxes could be 90%, which means is 90% of your retirement withdrawal will go to the federal government.”

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Cardone believes that taxes are such a threat to your retirement because they’re an expense that’s out of your control.

“Expenditures are a choice,” he said. “Taxes are a mandate. I don’t have a choice, and I don’t control how much it is. That pair of shoes, I could buy them or not buy them. The withholding tax, I cannot [control].”

If tax rates increase, even those who planned ahead and have millions of dollars saved may not be able to have a secure retirement.

“At 73 years old, you’re forced by the IRS to withdraw 4%, so for every million dollars [you have in retirement savings], you’re required to pull out $40,000 a year,” Cardone said. “If that’s taxed at 60%, you don’t have enough money to go to the retirement house.”

Given Cardone’s warning, you shouldn’t just plan for what you can control — you also need to prepare for what you can’t. That means building income streams that are tax-advantaged or tax-free, and staying flexible as laws change.

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This article originally appeared on GOBankingRates.com: Taxes Are the Biggest Threat to Your Retirement, According to Grant Cardone