VS error: Sequence contains more than one element

[craigktreasure]

Stack trace

System.InvalidOperationException : Sequence contains more than one element
   at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source)
   at async Treasure.Analyzers.MemberOrder.CodeFixes.MemberOrderCodeFixProvider.RegisterCodeFixesAsync(<Unknown Parameters>)
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at async Microsoft.CodeAnalysis.CodeFixes.CodeFixService.GetCodeFixesAsync(<Unknown Parameters>)
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at async Microsoft.CodeAnalysis.Extensions.IExtensionManagerExtensions.PerformFunctionAsync[T](<Unknown Parameters>)

I believe this happens if you have a sub-type that has errors in it.