[metapost] problem with DVItoMP

[Liunian]

hi,
   I am a new fish of metapost. When i use "label" command with "btex ... etx",
   such as:     
           glabel.bot(btex time etex ,OUT);

   an error occurred with a message:

         makempx: Command failed: dvitomp mpxerr.dvi start_w_l.mpx

         >> start_w_l.mp
         >> start_w_l.mpx
         ! Unable to make mpx file.
         l.42     label.bot(btex
                           time  etex ,OUT);

  I have check the file mpxerr.dvi, it can show the word "time" correctly.

  My metapost version is:
         This is MetaPost, Version 0.993 (Web2C 7.5.6)
  which with the "texlive 2007" on a ubuntu6.06 machine.

  Thanks for any clue. :)