diff --git a/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerManager.AnalyzerExecutionContext.cs b/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerManager.AnalyzerExecutionContext.cs
index fcbaa362fb8f2053dfa70b7f2c20789e0f038f00..2e9c14decccb5a36ea49a8590cf4f53f2d56fc29 100644
--- a/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerManager.AnalyzerExecutionContext.cs
+++ b/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerManager.AnalyzerExecutionContext.cs
@@ -139,7 +139,16 @@ HostSymbolStartAnalysisScope getSymbolAnalysisScopeCore()
                             lock (_gate)
                             {
                                 _lazyPendingMemberSymbolsMapOpt = _lazyPendingMemberSymbolsMapOpt ?? new Dictionary<ISymbol, HashSet<ISymbol>>();
-                                _lazyPendingMemberSymbolsMapOpt.Add(symbol, dependentSymbols);
+
+                                // Guard against entry added from another thread.
+                                if (!_lazyPendingMemberSymbolsMapOpt.ContainsKey(symbol))
+                                {
+                                    _lazyPendingMemberSymbolsMapOpt.Add(symbol, dependentSymbols);
+                                }
+                                else
+                                {
+                                    Debug.Assert(dependentSymbols.SetEquals(_lazyPendingMemberSymbolsMapOpt[symbol]));
+                                }
                             }
                         }