If you're trying to do this from the command line, you may be encountering an issue where a machine-wide environment variable 'Platform' is being set for you and working against you. I can reproduce this if I use the VS2012 Command window instead of a regular windows Command window.
At the command prompt type:
set platform
In a VS2012 Command window, I have a value of 'X64' preset. That seems to interfere with whatever is in my solution file.
In a regular Command window, the 'set' command results in a "variable not defined" message...which is good.
If the result of your 'set' command above returns no environment variable value, you should be good to go.