What does the error "the exec task needs a command to execute" mean?

Question

Ken Egozi has found an answer.

The message is caused by stray newline characters in pre or post build commands.

That link is no longer good.
HERE is a new link.

And here is a pertinent quote from the blog:
"After pooling out a few hairs, I remembers that I recently have edited the pre-build actions for this project. Looking this up, I found that I left a spare newline at the end of the pre-build script. So VS2005 sent msbuild an order to run an empty command."