Reserve Primary Fund on Tuesday became the first money market fund in 14 years to expose investors to losses as the fallout from Lehman Brothers’ bankruptcy spread to an investment class considered to be among the safest.

The Primary Fund, which had invested about $785m in debt securities issued by Lehman Brothers, suffered losses that forced the net value of its assets below $1 a share to 97 cents, according to Reserve Management Company, which manages the fund.