High-yield corporate bonds have been a hot investment in 2016. Now, some investors are fretting that the debt may have gotten too popular.

Drawn by higher yields than on safer bonds and lower valuations than on stocks, portfolio managers and individuals alike have poured money into junk bonds this year. In 2016, more than a net $6.4 billion had flowed into high-yield mutual funds through the end of August, according to data from Thomson Reuters Lipper. Over the prior three years, $47.7 billion flowed out of the funds.