The U.S. market's recent rally comes even as investors are putting far more money in bonds and foreign equities than domestic stocks.
That raises questions about the durability of the move higher and heightens the possibility of a significant leg lower, according to Michael Hartnett, chief investment strategist at Bank of America Merrill Lynch.
"Correction risks will grow in absence of fresh inflows in coming weeks, " Hartnett wrote in a note to clients. (Tweet this)
U.S. equity funds saw $6 billion in outflows over the past week, with a $4.9 billion chunk of that coming from exchange-traded funds, according to BofA.
Bond funds saw inflows of $5.6 billion continuing a major theme for the year. Fixed income has raked in $121 billion, while equity funds have attracted just $1 billion in 2015.