This bug was introduced in 5.2.1p2. Reverting to 5.2.1p1 fixes the issue.
The exception happens in MonoMethod.get_base_definition, after calling MemberInfo.IsDefined.
This is a bug that has already been reported and corrected. The fix should be in 5.2.1p3 when that release is available (hopefully in the next day or two).
Got the latest patch. Working great. Thanks.