`spack load` doesn't load right module when refined by dependency about spack HOT 2 CLOSED

$ spack find -vd parmetis
==> 2 installed packages.
-- linux-Ubuntu14-x86_64 / [email protected] ------------------------------
    [email protected]~debug~gdb+shared
        ^[email protected]~debug~gdb~idx64~real64+shared
        ^[email protected]+hydra+pmi~verbs

    [email protected]~debug~gdb+shared
        ^[email protected]~debug~gdb~idx64~real64+shared
        ^[email protected]~mxm~pmi~psm~psm2~slurm~sqlite3~thread_multiple~tm~verbs+vt
            ^[email protected]
                ^[email protected]
                    ^[email protected]

$ spack load parmetis 
==> Error: the constraint 'None' matches multiple packages, and this is not allowed in this context
[email protected]%[email protected]~debug~gdb+shared arch=linux-Ubuntu14-x86_64
[email protected]%[email protected]~debug~gdb+shared arch=linux-Ubuntu14-x86_64

$ spack load parmetis^openmpi

$ module list

Currently Loaded Modules:
  1) parmetis-4.0.3-gcc-4.8-i25qjy7