If your Assembly has included the

[assembly: CLSCompliantAttribute(true)]

attribute in it's AssemblyInfo, then you may encounter the above error.  It's not one of the most useful, given that you then 

have to scour the Common Language Specification rules to find out what exactly has gone wrong.

Even worse, is that it can send you in completely the wrong direction, as it did with me the other day.

If one assembly, which has the above attribute marked as true, refers to another assembly which does not include 

that attribute (or has it marked as false), then you will receive the above error message, telling you that Type such-and-such 

is not CLS-Compliant.  Even if it doesn't break any of the rules!

So, one to bear in mind for the future.