diff --git a/src/Workspaces/Core/Portable/Utilities/SpellChecker.cs b/src/Workspaces/Core/Portable/Utilities/SpellChecker.cs
index 1228363f56ea00b0a9af8f03251eebc1c5bfc1ed..f4438e12f0eb304b8058b527274639369690c31e 100644
--- a/src/Workspaces/Core/Portable/Utilities/SpellChecker.cs
+++ b/src/Workspaces/Core/Portable/Utilities/SpellChecker.cs
@@ -140,7 +140,10 @@ public void Free()
             _source = null;
             _editDistance = null;
             _lastAreSimilarResult = default(CacheResult);
-            s_pool.Push(this);
+            lock (s_poolGate)
+            {
+                s_pool.Push(this);
+            }
         }
 
         public static bool AreSimilar(string originalText, string candidateText)