VS2013 Code Analysis Custom Rules "CA0001 : A class that implements multiple rules must explicitly specify the warning identifier"

Moving from VS2010 to VS2013 we're running into this error message in our internal custom rules. I've tried debugging this but the error doesn't seem to actually be happening in our custom rules code. 

CA0001Error Running Code AnalysisCA0001 : A class that implements multiple rules must explicitly specify the warning identifier[Errors and Warnings](Global)

What does that error mean? I don't see any class in our custom rules that implements multiple rules but I also can't find any definition as to what that means. Each rule has exactly one Check() method for example and that plus the constructor are the only methods that override the interfaces that we inherit from.

Thank you