The iShares S&P National AMT-Free Bond Fund (NYSEArca: MUB) is up 2.5 percent in the past five days, its biggest advance since January 2011, according to Bloomberg.

MUB’s gains are outpacing the underlying debt in the fund due to money managers using it to mimic returns in the municipal market given a dearth of new issues coming onto the market, the Bloomberg report said.

That lack of liquidity in the municipal market is causing investors to hold on to their bonds and, as a result, the lack of supply is driving up the price of ETFs like MUB, according to the report.

Go to Bloomberg.com for the full breakdown.