diff --git a/src/Mediator.DependencyInjection/Builder/MediatorBuilder.Default.cs b/src/Mediator.DependencyInjection/Builder/MediatorBuilder.Default.cs
index 55e2749..e155c33 100644
--- a/src/Mediator.DependencyInjection/Builder/MediatorBuilder.Default.cs
+++ b/src/Mediator.DependencyInjection/Builder/MediatorBuilder.Default.cs
@@ -6,6 +6,12 @@ namespace Zapto.Mediator;
 
 public partial class MediatorBuilder : IMediatorBuilder
 {
+	public IMediatorBuilder AddDefaultRequestHandler(IDefaultRequestHandler handler)
+	{
+		_services.AddSingleton(handler);
+		return this;
+	}
+
 	public IMediatorBuilder AddDefaultRequestHandler(Type handlerType, RegistrationScope scope = RegistrationScope.Transient)
 	{
 		_services.Add(new ServiceDescriptor(typeof(IDefaultRequestHandler), handlerType, GetLifetime(scope)));
@@ -18,6 +24,12 @@ public IMediatorBuilder AddDefaultRequestHandler<THandler>(RegistrationScope sco
 		return this;
 	}
 
+	public IMediatorBuilder AddDefaultNotificationHandler(IDefaultNotificationHandler handler)
+	{
+		_services.AddSingleton(handler);
+		return this;
+	}
+
 	public IMediatorBuilder AddDefaultNotificationHandler(Type handlerType, RegistrationScope scope = RegistrationScope.Transient)
 	{
 		_services.Add(new ServiceDescriptor(typeof(IDefaultNotificationHandler), handlerType, GetLifetime(scope)));
@@ -30,6 +42,12 @@ public IMediatorBuilder AddDefaultNotificationHandler<THandler>(RegistrationScop
 		return this;
 	}
 
+	public IMediatorBuilder AddDefaultStreamRequestHandler(IDefaultStreamRequestHandler handler)
+	{
+		_services.AddSingleton(handler);
+		return this;
+	}
+
 	public IMediatorBuilder AddDefaultStreamRequestHandler(Type handlerType, RegistrationScope scope = RegistrationScope.Transient)
 	{
 		_services.Add(new ServiceDescriptor(typeof(IDefaultStreamRequestHandler), handlerType, GetLifetime(scope)));
diff --git a/src/Mediator/Exceptions/HandlerNotFoundException.cs b/src/Mediator/Exceptions/HandlerNotFoundException.cs
index fae9af6..bdf4099 100644
--- a/src/Mediator/Exceptions/HandlerNotFoundException.cs
+++ b/src/Mediator/Exceptions/HandlerNotFoundException.cs
@@ -3,7 +3,7 @@
 
 namespace Zapto.Mediator;
 
-public class HandlerNotFoundException : Exception
+public class HandlerNotFoundException : InvalidOperationException
 {
 	public HandlerNotFoundException()
 	{
diff --git a/src/Mediator/IMediatorBuilder.cs b/src/Mediator/IMediatorBuilder.cs
index 529bd74..f20b5d8 100644
--- a/src/Mediator/IMediatorBuilder.cs
+++ b/src/Mediator/IMediatorBuilder.cs
@@ -119,14 +119,20 @@ IMediatorBuilder AddStreamRequestHandler<TRequest, TResponse>(IStreamRequestHand
     public IMediatorBuilder AddStreamRequestHandler<TRequest, TResponse>(Func<IServiceProvider, TRequest, IAsyncEnumerable<TResponse>> handler)
         where TRequest : IStreamRequest<TResponse>;
 
+    IMediatorBuilder AddDefaultRequestHandler(IDefaultRequestHandler handler);
+
     IMediatorBuilder AddDefaultRequestHandler(Type handlerType, RegistrationScope scope = RegistrationScope.Transient);
 
     IMediatorBuilder AddDefaultRequestHandler<THandler>(RegistrationScope scope = RegistrationScope.Transient) where THandler : class, IDefaultRequestHandler;
 
+    IMediatorBuilder AddDefaultNotificationHandler(IDefaultNotificationHandler handler);
+
     IMediatorBuilder AddDefaultNotificationHandler(Type handlerType, RegistrationScope scope = RegistrationScope.Transient);
 
     IMediatorBuilder AddDefaultNotificationHandler<THandler>(RegistrationScope scope = RegistrationScope.Transient) where THandler : class, IDefaultNotificationHandler;
 
+    IMediatorBuilder AddDefaultStreamRequestHandler(IDefaultStreamRequestHandler handler);
+
     IMediatorBuilder AddDefaultStreamRequestHandler(Type handlerType, RegistrationScope scope = RegistrationScope.Transient);
 
     IMediatorBuilder AddDefaultStreamRequestHandler<THandler>(RegistrationScope scope = RegistrationScope.Transient) where THandler : class, IDefaultStreamRequestHandler;
diff --git a/tests/Mediator.DependencyInjection.Tests/DefaultHandler/NotificationDefaultHandlerTest.cs b/tests/Mediator.DependencyInjection.Tests/DefaultHandler/NotificationDefaultHandlerTest.cs
new file mode 100644
index 0000000..c793965
--- /dev/null
+++ b/tests/Mediator.DependencyInjection.Tests/DefaultHandler/NotificationDefaultHandlerTest.cs
@@ -0,0 +1,29 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.DependencyInjection;
+using NSubstitute;
+using Xunit;
+using Zapto.Mediator;
+
+namespace Mediator.DependencyInjection.Tests.DefaultHandler;
+
+public class NotificationDefaultHandlerTest
+{
+	[Fact]
+	public async Task TestNotification()
+	{
+		var handler = Substitute.For<IDefaultNotificationHandler>();
+
+		var serviceProvider = new ServiceCollection()
+			.AddMediator(b => b.AddDefaultNotificationHandler(handler))
+			.BuildServiceProvider();
+
+		var mediator = serviceProvider.GetRequiredService<IMediator>();
+
+		await mediator.Publish(new Notification());
+
+		_ = handler.Received()
+			.Handle(Arg.Any<IServiceProvider>(), Arg.Any<Notification>(), Arg.Any<CancellationToken>());
+	}
+}
diff --git a/tests/Mediator.DependencyInjection.Tests/DefaultHandler/RequestDefaultHandlerTest.cs b/tests/Mediator.DependencyInjection.Tests/DefaultHandler/RequestDefaultHandlerTest.cs
new file mode 100644
index 0000000..e5a11c7
--- /dev/null
+++ b/tests/Mediator.DependencyInjection.Tests/DefaultHandler/RequestDefaultHandlerTest.cs
@@ -0,0 +1,29 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.DependencyInjection;
+using NSubstitute;
+using Xunit;
+using Zapto.Mediator;
+
+namespace Mediator.DependencyInjection.Tests.DefaultHandler;
+
+public class RequestDefaultHandlerTest
+{
+	[Fact]
+	public async Task TestRequest()
+	{
+		var handler = Substitute.For<IDefaultRequestHandler>();
+
+		var serviceProvider = new ServiceCollection()
+			.AddMediator(b => b.AddDefaultRequestHandler(handler))
+			.BuildServiceProvider();
+
+		var mediator = serviceProvider.GetRequiredService<IMediator>();
+
+		await mediator.Send<Request, int>(new Request());
+
+		_ = handler.Received()
+			.Handle<Request, int>(Arg.Any<IServiceProvider>(), Arg.Any<Request>(), Arg.Any<CancellationToken>());
+	}
+}
diff --git a/tests/Mediator.DependencyInjection.Tests/DefaultHandler/StreamRequestDefaultHandlerTest.cs b/tests/Mediator.DependencyInjection.Tests/DefaultHandler/StreamRequestDefaultHandlerTest.cs
new file mode 100644
index 0000000..fa244d7
--- /dev/null
+++ b/tests/Mediator.DependencyInjection.Tests/DefaultHandler/StreamRequestDefaultHandlerTest.cs
@@ -0,0 +1,35 @@
+using System;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.DependencyInjection;
+using NSubstitute;
+using Xunit;
+using Zapto.Mediator;
+
+namespace Mediator.DependencyInjection.Tests.DefaultHandler;
+
+public class StreamRequestDefaultHandlerTest
+{
+	[Fact]
+	public async Task TestStream()
+	{
+		var handler = Substitute.For<IDefaultStreamRequestHandler>();
+
+		handler.Handle<StreamRequest, int>(Arg.Any<IServiceProvider>(), Arg.Any<StreamRequest>(), Arg.Any<CancellationToken>())
+			.Returns(Array.Empty<int>().ToAsyncEnumerable());
+
+		var serviceProvider = new ServiceCollection()
+			.AddMediator(b => b.AddDefaultStreamRequestHandler(handler))
+			.BuildServiceProvider();
+
+		var mediator = serviceProvider.GetRequiredService<IMediator>();
+
+		await mediator
+			.CreateStream<StreamRequest, int>(new StreamRequest())
+			.ToListAsync();
+
+		_ = handler.Received()
+			.Handle<StreamRequest, int>(Arg.Any<IServiceProvider>(), Arg.Any<StreamRequest>(), Arg.Any<CancellationToken>());
+	}
+}
diff --git a/tests/Mediator.DependencyInjection.Tests/RequestTest.cs b/tests/Mediator.DependencyInjection.Tests/RequestTest.cs
index fe0dc98..30b59af 100644
--- a/tests/Mediator.DependencyInjection.Tests/RequestTest.cs
+++ b/tests/Mediator.DependencyInjection.Tests/RequestTest.cs
@@ -109,4 +109,30 @@ public async Task TestCollection()
 
         Assert.Equal(0, result.Count);
     }
+
+    [Fact]
+    public async Task TestNoHandler()
+    {
+        var serviceProvider = new ServiceCollection()
+            .AddMediator(_ => {})
+            .BuildServiceProvider();
+
+        var mediator = serviceProvider.GetRequiredService<IMediator>();
+
+        await Assert.ThrowsAsync<HandlerNotFoundException>(() => mediator.Send<Request, int>(new Request()).AsTask());
+    }
+
+    [Fact]
+    public async Task TestNoHandlerNamespace()
+    {
+        var ns = new MediatorNamespace("test");
+
+        var serviceProvider = new ServiceCollection()
+            .AddMediator(_ => {})
+            .BuildServiceProvider();
+
+        var mediator = serviceProvider.GetRequiredService<IMediator>();
+
+        await Assert.ThrowsAsync<NamespaceHandlerNotFoundException>(() => mediator.Send<Request, int>(ns, new Request()).AsTask());
+    }
 }
diff --git a/tests/Mediator.DependencyInjection.Tests/StreamRequestTest.cs b/tests/Mediator.DependencyInjection.Tests/StreamRequestTest.cs
index cfe2aa6..a6b9193 100644
--- a/tests/Mediator.DependencyInjection.Tests/StreamRequestTest.cs
+++ b/tests/Mediator.DependencyInjection.Tests/StreamRequestTest.cs
@@ -111,4 +111,33 @@ await mediator
         _ = handler.Received(requiredNumberOfCalls: 2)
             .Handle(Arg.Any<IServiceProvider>(), Arg.Any<StreamRequest>(), Arg.Any<CancellationToken>());
     }
+
+    [Fact]
+    public async Task TestNoHandler()
+    {
+        var serviceProvider = new ServiceCollection()
+            .AddMediator(_ => { })
+            .BuildServiceProvider();
+
+        var mediator = serviceProvider.GetRequiredService<IMediator>();
+
+        await Assert.ThrowsAsync<HandlerNotFoundException>(() => mediator
+            .CreateStream<StreamRequest, int>(new StreamRequest())
+            .ToListAsync().AsTask());
+    }
+
+    [Fact]
+    public async Task TestNoHandlerNamespace()
+    {
+        var ns = new MediatorNamespace("test");
+        var serviceProvider = new ServiceCollection()
+            .AddMediator(_ => { })
+            .BuildServiceProvider();
+
+        var mediator = serviceProvider.GetRequiredService<IMediator>();
+
+        await Assert.ThrowsAsync<NamespaceHandlerNotFoundException>(() => mediator
+            .CreateStream<StreamRequest, int>(ns, new StreamRequest())
+            .ToListAsync().AsTask());
+    }
 }