XML document is invalid, likely due to invalid characters

I have a number of feeds working fine with this plugin. I am getting the following error when I test a particular feed however:

"XML document is invalid, likely due to invalid characters. XML error: undefined entity at line 294, column 85"

I'd like to at least give instructions to the source if that is the problem. Any help would be appreciated. The feed in question works fine just using a browser or Google Reader.

Thanks in advance.