Bond traders have been pulling out of investment-grade debt, potentially signalling trouble ahead for fixed-income exchange traded funds.

According to Wells Fargo data, investors pulled $1.1 billion from U.S. investment-grade bond funds last week, the largest withdrawal since 2013, reports Lisa Abramowicz for Bloomberg.

Meanwhile, the iShares iBoxx $ Investment Grade Corporate Bond ETF (NYSEArca: LQD) saw $554.5 million in net asset outflows and Vanguard Intermediate-Term Corporate Bond ETF (NYSEArca: VCIT) lost $68.1 million last week, according to Over the past week, LQD experienced an additional $531.2 million in outflows.

“Credit is the warning signal that everyone’s been looking for,” Jim Bianco, founder of Bianco Research LLC, told Bloomberg. “That is something that’s been a very good leading indicator for the past 15 years.”

Fixed-income investors are growing wary of investment-grade debt with yields at historically low 3.4% while companies are increasing acquisitions, share buybacks an dividends. Additionally, many bond traders are cautious ahead of the Federal Reserve’s planned interest rate hike, its first since 2006.

Consequently, corporate-bond investors are demanding a 1.64 percentage point premium above benchmark government rates for investment-grade notes, the highest since July 2013. For instance, LQD has a 3.59% 30-day SEC yield, compared to yields of 2.184% for benchmark 10-year notes.