diff --git a/src/Lamar/IoC/Instances/GeneratedInstance.cs b/src/Lamar/IoC/Instances/GeneratedInstance.cs
index 405e179b..33c9b45c 100644
--- a/src/Lamar/IoC/Instances/GeneratedInstance.cs
+++ b/src/Lamar/IoC/Instances/GeneratedInstance.cs
@@ -98,7 +98,7 @@ public Func<Scope, object> BuildFuncResolver(Scope scope)
                         }
 
                         service = resolver(s);
-                        s.Services.Add(Hash, service);
+                        s.Services.TryAdd(Hash, service);
 
                         return service;
                     }
@@ -126,7 +126,7 @@ public Func<Scope, object> BuildFuncResolver(Scope scope)
                         }
 
                         service = resolver(root);
-                        root.Services.Add(Hash, service);
+                        root.Services.TryAdd(Hash, service);
 
                         return service;
                     }
diff --git a/src/Lamar/IoC/Instances/Instance.cs b/src/Lamar/IoC/Instances/Instance.cs
index 49de677d..e793a995 100644
--- a/src/Lamar/IoC/Instances/Instance.cs
+++ b/src/Lamar/IoC/Instances/Instance.cs
@@ -234,9 +234,9 @@ protected bool tryGetService(Scope scope, out object service)
             return scope.Services.TryGetValue(Hash, out service);
         }
 
-        protected void store(Scope scope, object service)
+        protected bool store(Scope scope, object service)
         {
-            scope.Services.Add(Hash, service);
+            return scope.Services.TryAdd(Hash, service);
         }
 
         /// <summary>
diff --git a/src/Lamar/IoC/Resolvers/ScopedResolver.cs b/src/Lamar/IoC/Resolvers/ScopedResolver.cs
index 58b6a75c..fdd6370f 100644
--- a/src/Lamar/IoC/Resolvers/ScopedResolver.cs
+++ b/src/Lamar/IoC/Resolvers/ScopedResolver.cs
@@ -23,7 +23,7 @@ public object Resolve(Scope scope)
                 }
                 
                 service = (T) Build(scope);
-                scope.Services.Add(Hash, service);
+                scope.Services.TryAdd(Hash, service);
 
                 if (service is IDisposable)
                 {
diff --git a/src/Lamar/IoC/Scope.cs b/src/Lamar/IoC/Scope.cs
index 3c6f8fdb..06b64f57 100644
--- a/src/Lamar/IoC/Scope.cs
+++ b/src/Lamar/IoC/Scope.cs
@@ -93,7 +93,7 @@ protected void assertNotDisposed()
 
         public ConcurrentBag<IDisposable> Disposables { get; } = new ConcurrentBag<IDisposable>();
 
-        internal readonly Dictionary<int, object> Services = new Dictionary<int, object>();
+        internal readonly ConcurrentDictionary<int, object> Services = new ConcurrentDictionary<int, object>();
 
         public virtual void Dispose()
         {