KeyNotFoundException thrown from FindSymbols.FindReferencesSearchEngine.ProcessDocumentAsync

[tmat]

While working in Roslyn.sln. Not sure what exactly triggered it.

StreamJsonRpc.RemoteInvocationException: The given key 'Public ReadOnly Property Method As Microsoft.CodeAnalysis.Symbols.IMethodSymbolInternal' was not present in the dictionary.
   at StreamJsonRpc.JsonRpc.<InvokeCoreAsync>d__151`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.Remote.BrokeredServiceConnection`1.<TryInvokeAsync>d__21.MoveNext()
RPC server exception:

System.Collections.Generic.KeyNotFoundException: The given key 'Public ReadOnly Property Method As Microsoft.CodeAnalysis.Symbols.IMethodSymbolInternal' was not present in the dictionary.
      at System.Collections.Concurrent.ConcurrentDictionary`2.ThrowKeyNotFoundException(TKey key)
      at System.Collections.Concurrent.ConcurrentDictionary`2.get_Item(TKey key)
      at Microsoft.CodeAnalysis.FindSymbols.FindReferencesSearchEngine.<>c__DisplayClass20_0.<<ProcessDocumentAsync>g__ProcessDocumentAsync|0>d.MoveNext()
   --- End of stack trace from previous location ---
      at Microsoft.CodeAnalysis.FindSymbols.FindReferencesSearchEngine.ProcessDocumentAsync(Document document, MetadataUnifyingSymbolHashSet symbols, Dictionary`2 symbolToGlobalAliases, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.FindSymbols.FindReferencesSearchEngine.ProcessDocumentAsync(Document document, MetadataUnifyingSymbolHashSet symbols, Dictionary`2 symbolToGlobalAliases, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.FindSymbols.FindReferencesSearchEngine.ProcessProjectAsync(Project project, ImmutableArray`1 allSymbols, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.FindSymbols.FindReferencesSearchEngine.ProcessProjectAsync(Project project, ImmutableArray`1 allSymbols, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.FindSymbols.FindReferencesSearchEngine.FindReferencesAsync(ImmutableArray`1 symbols, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.FindSymbols.FindReferencesSearchEngine.FindReferencesAsync(ImmutableArray`1 symbols, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.FindSymbols.FindReferencesSearchEngine.FindReferencesAsync(ImmutableArray`1 symbols, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.FindSymbols.SymbolFinder.FindReferencesAsync(ISymbol symbol, Solution solution, IStreamingFindReferencesProgress progress, IImmutableSet`1 documents, FindReferencesSearchOptions options, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.FindUsages.AbstractFindUsagesService.FindReferencesAsync(IFindUsagesContext context, ISymbol symbol, Project project, FindReferencesSearchOptions options, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.Remote.RemoteFindUsagesService.<>c__DisplayClass3_0.<<FindReferencesAsync>b__0>d.MoveNext()
   --- End of stack trace from previous location ---
      at Microsoft.CodeAnalysis.Remote.BrokeredServiceBase.<>c__DisplayClass16_0.<<RunServiceAsync>b__1>d.MoveNext()
   --- End of stack trace from previous location ---
      at Microsoft.CodeAnalysis.Remote.RemoteWorkspace.<>c__DisplayClass9_0`1.<<RunWithSolutionAsync>g__ProcessSolutionAsync|1>d.MoveNext()
   --- End of stack trace from previous location ---
      at Microsoft.CodeAnalysis.Remote.RemoteWorkspace.RunWithSolutionAsync[T](AssetProvider assetProvider, Checksum solutionChecksum, Int32 workspaceVersion, Boolean updatePrimaryBranch, Func`2 implementation, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.Remote.RemoteWorkspace.RunWithSolutionAsync[T](AssetProvider assetProvider, Checksum solutionChecksum, Int32 workspaceVersion, Boolean updatePrimaryBranch, Func`2 implementation, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.Remote.BrokeredServiceBase.RunWithSolutionAsync[T](Checksum solutionChecksum, Func`2 implementation, CancellationToken cancellationToken)
      at Microsoft.CodeAnalysis.Remote.BrokeredServiceBase.<>c__DisplayClass16_0.<<RunServiceAsync>b__0>d.MoveNext()
   --- End of stack trace from previous location ---
      at Microsoft.CodeAnalysis.Remote.BrokeredServiceBase.RunServiceImplAsync(Func`2 implementation, CancellationToken cancellationToken)