A Minneapolis hedge fund has halted withdrawals following a huge margin call.

Whitebox Advisors is suspending redemptions after Goldman Sachs doubled its margin requirements on the hedge fund’s convertible bond portfolio, Hedge Fund Alert reports. Whitebox met the demand by putting more of its own capital and devalued holdings up as collateral, leaving it without enough liquidity to meet redemption requests.

The $4 billion firm was drafting a letter to investors to explain the situation, HFA reports, as well as to lay out the terms under with clients can get some of their capital out of the fund.

Whitebox, which enjoyed double-digit returns last year, has not been so lucky this year: Two of its funds are in the red year-to-date.