Activation Error: The plugin generated 143 characters of unexpected output during activation.

When activating MU Support System 1.6.7 I get the following error

The plugin generated 143 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

I have WP 3.2.1 Network and Multi-DB plugin. I'm using the default theme.