Municipal bonds, along with related exchange traded funds, are trading at their deepest discount to U.S. Treasuries in four months after another round of debt concerns over Puerto Rico’s finances sparked a sell-off in muni funds.

High-yield muni ETFs suffered the brunt of the blow, with the Market Vectors High Yield Municipal Index ETF (NYSEArca: HYD) down 2.4% and SPDR Nuveen S&P High Yield Municipal Bond ETF (NYSEArca: HYMB) 1.1% lower over the past month.

Meanwhile, investment-grade muni ETFs iShares National AMT-Free Muni Bond ETF (NYSEArca: MUB) and SPDR Nuveen Barclays Municipal Bond ETF (NYSEArca: TFI) were up 0.5% and 0.3% over the past month, respectively.

Benchmark 10-year muni yields were hovering around 2.44% while 10-year Treasuries were at around 2.51%, making the ratio between the two yields about 97%, the highest since March 13, which also indicated that local debt has cheapened on a relative basis, reports Brian Chappatta for Bloomberg.

“As munis begin to cheapen up and Treasuries stay stable or rally, at some point people are going to say munis are a good relative value,” Dan Toboja, senior vice president of Ziegler Capital Markets, said in the article.

Toboja also believes that investors may return to the relatively higher tax-free yields in munis as supply next week is set to come in at about $4.7 billion, compared to the 2014 average of $5.4 billion. [Low Supply to Support Municipal Bonds Market, ETFs]