[oe] Why can't find preferred version?

[代尔欣]

Hello，
      Met below problems：
      Folder "packages/libnl" have 3 bb files:
      libnl_1.0-pre6.bb / libnl_1.0-pre8.bb / libnl_0.5.0.bb

      I want to use libnl_1.0-pre6.bb. I add below line into .conf file:
      PREFERRED_VERSION_libnl ?= "1.0-pre6"

     But when compile, the message：
     NOTE: preferred version 1.0-pre6 of libnl not available (for item
libnl)

     pop, and the OE use libnl_1.0-pre8.bb. Why?

     Thanks!