diff --git a/Source/EntityFramework.Extended/Dynamic/DynamicQueryable.cs b/Source/EntityFramework.Extended/Dynamic/DynamicQueryable.cs
index 20dced3..9620b93 100644
--- a/Source/EntityFramework.Extended/Dynamic/DynamicQueryable.cs
+++ b/Source/EntityFramework.Extended/Dynamic/DynamicQueryable.cs
@@ -285,8 +285,20 @@ public Type GetDynamicClass(IEnumerable<DynamicProperty> properties)
                 Type type;
                 if (!classes.TryGetValue(signature, out type))
                 {
-                    type = CreateDynamicClass(signature.properties);
-                    classes.Add(signature, type);
+                    // Upgrade the lock, and double check the dictionary to improve thread safety.
+                    LockCookie cookie = rwLock.UpgradeToWriterLock(Timeout.Infinite);
+                    try
+                    {
+                        if (!classes.TryGetValue(signature, out type))
+                        {
+                            type = CreateDynamicClass(signature.properties);
+                            classes.Add(signature, type);
+                        }
+                    }
+                    finally
+                    {
+                        rwLock.DowngradeFromWriterLock(ref cookie);
+                    }
                 }
                 return type;
             }
@@ -298,34 +310,26 @@ public Type GetDynamicClass(IEnumerable<DynamicProperty> properties)
 
         Type CreateDynamicClass(DynamicProperty[] properties)
         {
-            LockCookie cookie = rwLock.UpgradeToWriterLock(Timeout.Infinite);
-            try
-            {
-                string typeName = "DynamicClass" + (classCount + 1);
+            string typeName = "DynamicClass" + (classCount + 1);
 #if ENABLE_LINQ_PARTIAL_TRUST
                 new ReflectionPermission(PermissionState.Unrestricted).Assert();
 #endif
-                try
-                {
-                    TypeBuilder tb = this.module.DefineType(typeName, TypeAttributes.Class |
-                        TypeAttributes.Public, typeof(DynamicClass));
-                    FieldInfo[] fields = GenerateProperties(tb, properties);
-                    GenerateEquals(tb, fields);
-                    GenerateGetHashCode(tb, fields);
-                    Type result = tb.CreateType();
-                    classCount++;
-                    return result;
-                }
-                finally
-                {
-#if ENABLE_LINQ_PARTIAL_TRUST
-                    PermissionSet.RevertAssert();
-#endif
-                }
+            try
+            {
+                TypeBuilder tb = this.module.DefineType(typeName, TypeAttributes.Class |
+                    TypeAttributes.Public, typeof(DynamicClass));
+                FieldInfo[] fields = GenerateProperties(tb, properties);
+                GenerateEquals(tb, fields);
+                GenerateGetHashCode(tb, fields);
+                Type result = tb.CreateType();
+                classCount++;
+                return result;
             }
             finally
             {
-                rwLock.DowngradeFromWriterLock(ref cookie);
+#if ENABLE_LINQ_PARTIAL_TRUST
+                    PermissionSet.RevertAssert();
+#endif
             }
         }