Mathtime and prosper

[Jeremy Avigad]

I am trying to use the mathtime fonts with the prosper slide package under
Miktex. The following trivial tex file

  \documentclass{prosper}

  \usepackage{mathtime}

  \begin{document}

  \end{document}

yields the following error message:

  ERROR: LaTeX Error: This NFSS system isn't set up properly.

  --- TeX said ---

  See the LaTeX manual or LaTeX Companion for explanation.
  Type  H <return>  for immediate help.
   ...

  l.5 \begin{document}

  --- HELP ---
  From the .log file...

  For encoding scheme TS1 the defaults helvetica/m/n do not form a valid
  font shape

I have no trouble, for example, using the txfonts package.

Am I doing something wrong? Is there a fix?

Thanks,

Jeremy Avigad