diff --git a/src/VisualStudio/Core/Def/Implementation/TaskList/ExternalErrorDiagnosticUpdateSource.cs b/src/VisualStudio/Core/Def/Implementation/TaskList/ExternalErrorDiagnosticUpdateSource.cs
index 00c67861e94aa..4b2cecef0baca 100644
--- a/src/VisualStudio/Core/Def/Implementation/TaskList/ExternalErrorDiagnosticUpdateSource.cs
+++ b/src/VisualStudio/Core/Def/Implementation/TaskList/ExternalErrorDiagnosticUpdateSource.cs
@@ -420,26 +420,29 @@ public void Done()
 
             public bool SupportedDiagnosticId(ProjectId projectId, string id)
             {
-                if (_diagnosticIdMap.TryGetValue(projectId, out var ids))
+                lock (_diagnosticIdMap)
                 {
-                    return ids.Contains(id);
-                }
+                    if (_diagnosticIdMap.TryGetValue(projectId, out var ids))
+                    {
+                        return ids.Contains(id);
+                    }
 
-                // set ids set
-                var map = new HashSet<string>();
-                _diagnosticIdMap.Add(projectId, map);
+                    // set ids set
+                    var map = new HashSet<string>();
+                    _diagnosticIdMap.Add(projectId, map);
 
-                var project = _solution.GetProject(projectId);
-                if (project == null)
-                {
-                    // projectId no longer exist, return false;
-                    return false;
-                }
+                    var project = _solution.GetProject(projectId);
+                    if (project == null)
+                    {
+                        // projectId no longer exist, return false;
+                        return false;
+                    }
 
-                var descriptorMap = _owner._diagnosticService.GetDiagnosticDescriptors(project);
-                map.UnionWith(descriptorMap.Values.SelectMany(v => v.Select(d => d.Id)));
+                    var descriptorMap = _owner._diagnosticService.GetDiagnosticDescriptors(project);
+                    map.UnionWith(descriptorMap.Values.SelectMany(v => v.Select(d => d.Id)));
 
-                return map.Contains(id);
+                    return map.Contains(id);
+                }
             }
 
             public ImmutableArray<DiagnosticData> GetBuildDiagnostics()
diff --git a/src/VisualStudio/Core/Test/Diagnostics/ExternalDiagnosticUpdateSourceTests.vb b/src/VisualStudio/Core/Test/Diagnostics/ExternalDiagnosticUpdateSourceTests.vb
index c1870799b2c95..6d86109077a0d 100644
--- a/src/VisualStudio/Core/Test/Diagnostics/ExternalDiagnosticUpdateSourceTests.vb
+++ b/src/VisualStudio/Core/Test/Diagnostics/ExternalDiagnosticUpdateSourceTests.vb
@@ -68,6 +68,20 @@ Namespace Microsoft.VisualStudio.LanguageServices.UnitTests.Diagnostics
             End Using
         End Sub
 
+        <Fact>
+        Public Sub TestExternalDiagnostics_SupportedDiagnosticId_Concurrent()
+            Using workspace = TestWorkspace.CreateCSharp(String.Empty)
+                Dim waiter = New Waiter()
+                Dim service = New TestDiagnosticAnalyzerService()
+                Dim source = New ExternalErrorDiagnosticUpdateSource(workspace, service, New MockDiagnosticUpdateSourceRegistrationService(), waiter)
+
+                Dim project = workspace.CurrentSolution.Projects.First()
+                source.OnSolutionBuild(Me, Shell.UIContextChangedEventArgs.From(True))
+
+                Parallel.For(0, 100, Sub(i As Integer) source.SupportedDiagnosticId(project.Id, "CS1002"))
+            End Using
+        End Sub
+
         <Fact>
         Public Async Function TestExternalDiagnostics_DuplicatedError() As Task
             Using workspace = TestWorkspace.CreateCSharp(String.Empty)