diff --git a/TUnit.Mocks.SourceGenerator.Tests/MockGeneratorTests.cs b/TUnit.Mocks.SourceGenerator.Tests/MockGeneratorTests.cs
index 8e680cd69d..6c1906756d 100644
--- a/TUnit.Mocks.SourceGenerator.Tests/MockGeneratorTests.cs
+++ b/TUnit.Mocks.SourceGenerator.Tests/MockGeneratorTests.cs
@@ -133,6 +133,61 @@ void M()
         return VerifyGeneratorOutput(source);
     }
 
+    [Test]
+    public Task Interface_With_Nullable_Event_Args()
+    {
+        // Regression test for https://github.com/thomhurst/TUnit/issues/5425
+        // Nullability of generic event handler type arguments must be preserved
+        // in the generated mock to avoid CS8604.
+        var source = """
+            #nullable enable
+            using System;
+            using TUnit.Mocks;
+
+            public interface IFoo
+            {
+                event EventHandler<string?> Something;
+            }
+
+            public class TestUsage
+            {
+                void M()
+                {
+                    var mock = Mock.Of<IFoo>();
+                }
+            }
+            """;
+
+        return VerifyGeneratorOutput(source);
+    }
+
+    [Test]
+    public Task Interface_With_Nullable_Event_And_Nullable_Args()
+    {
+        // Combined regression for #5424 + #5425: both the delegate itself and
+        // its generic type argument are nullable (`EventHandler<string?>?`).
+        var source = """
+            #nullable enable
+            using System;
+            using TUnit.Mocks;
+
+            public interface IFoo
+            {
+                event EventHandler<string?>? Something;
+            }
+
+            public class TestUsage
+            {
+                void M()
+                {
+                    var mock = Mock.Of<IFoo>();
+                }
+            }
+            """;
+
+        return VerifyGeneratorOutput(source);
+    }
+
     [Test]
     public Task Interface_With_Multiple_Multi_Parameter_Events()
     {
diff --git a/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Nullable_Event_And_Nullable_Args.verified.txt b/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Nullable_Event_And_Nullable_Args.verified.txt
new file mode 100644
index 0000000000..382753abce
--- /dev/null
+++ b/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Nullable_Event_And_Nullable_Args.verified.txt
@@ -0,0 +1,160 @@
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    public sealed class IFooMock : global::TUnit.Mocks.Mock<global::IFoo>, global::IFoo
+    {
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        internal IFooMock(global::IFoo mockObject, global::TUnit.Mocks.MockEngine<global::IFoo> engine)
+            : base(mockObject, engine) { }
+
+        event global::System.EventHandler<string?>? global::IFoo.Something { add => Object.Something += value; remove => Object.Something -= value; }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+    public readonly struct IFoo_MockEvents
+    {
+        internal readonly global::TUnit.Mocks.MockEngine<global::IFoo> Engine;
+
+        internal IFoo_MockEvents(global::TUnit.Mocks.MockEngine<global::IFoo> engine) => Engine = engine;
+    }
+
+    public static class IFoo_MockEventsExtensions
+    {
+        extension(global::TUnit.Mocks.Mock<global::IFoo> mock)
+        {
+            public IFoo_MockEvents Events => new(global::TUnit.Mocks.MockRegistry.GetEngine(mock));
+        }
+
+        extension(IFoo_MockEvents events)
+        {
+            public global::TUnit.Mocks.EventSubscriptionAccessor Something
+                => new(events.Engine, "Something");
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    file sealed class IFooMockImpl : global::IFoo, global::TUnit.Mocks.IRaisable, global::TUnit.Mocks.IMockObject
+    {
+        private readonly global::TUnit.Mocks.MockEngine<global::IFoo> _engine;
+
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        global::TUnit.Mocks.IMock? global::TUnit.Mocks.IMockObject.MockWrapper { get; set; }
+
+        internal IFooMockImpl(global::TUnit.Mocks.MockEngine<global::IFoo> engine)
+        {
+            _engine = engine;
+        }
+
+        private global::System.EventHandler<string?>? _backing_Something;
+
+        public event global::System.EventHandler<string?>? Something
+        {
+            add { _backing_Something += value; _engine.RecordEventSubscription("Something", true); }
+            remove { _backing_Something -= value; _engine.RecordEventSubscription("Something", false); }
+        }
+
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        internal void Raise_Something(string? e)
+        {
+            _backing_Something?.Invoke(this, e);
+        }
+
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        public void RaiseEvent(string eventName, object? args)
+        {
+            switch (eventName)
+            {
+                case "Something":
+                    {
+                        Raise_Something((string?)args!);
+                        break;
+                    }
+                default:
+                    throw new global::System.InvalidOperationException($"No event named '{eventName}' exists on this mock.");
+            }
+        }
+    }
+
+    file static class IFooMockFactory
+    {
+        [global::System.Runtime.CompilerServices.ModuleInitializer]
+        internal static void Register()
+        {
+            global::TUnit.Mocks.MockRegistry.RegisterFactory<global::IFoo>(Create);
+        }
+
+        internal static global::TUnit.Mocks.Mock<global::IFoo> Create(global::TUnit.Mocks.MockBehavior behavior, object[] constructorArgs)
+        {
+            if (constructorArgs.Length > 0) throw new global::System.ArgumentException($"Interface mock 'global::IFoo' does not support constructor arguments, but {constructorArgs.Length} were provided.");
+            var engine = new global::TUnit.Mocks.MockEngine<global::IFoo>(behavior);
+            var impl = new IFooMockImpl(engine);
+            engine.Raisable = impl;
+            var mock = new IFooMock(impl, engine);
+            return mock;
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    public static class IFoo_MockMemberExtensions
+    {
+        public static void RaiseSomething(this global::TUnit.Mocks.Mock<global::IFoo> mock, string? e)
+        {
+            ((global::TUnit.Mocks.IRaisable)global::TUnit.Mocks.MockRegistry.GetEngine(mock).Raisable!).RaiseEvent("Something", (object?)e);
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks
+{
+    public static class IFoo_MockStaticExtension
+    {
+        extension(global::IFoo)
+        {
+            public static global::TUnit.Mocks.Generated.IFooMock Mock(global::TUnit.Mocks.MockBehavior behavior = global::TUnit.Mocks.MockBehavior.Loose)
+            {
+                return (global::TUnit.Mocks.Generated.IFooMock)global::TUnit.Mocks.Mock.Of<global::IFoo>(behavior);
+            }
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated;
\ No newline at end of file
diff --git a/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Nullable_Event_Args.verified.txt b/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Nullable_Event_Args.verified.txt
new file mode 100644
index 0000000000..44d26a5fb2
--- /dev/null
+++ b/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Nullable_Event_Args.verified.txt
@@ -0,0 +1,160 @@
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    public sealed class IFooMock : global::TUnit.Mocks.Mock<global::IFoo>, global::IFoo
+    {
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        internal IFooMock(global::IFoo mockObject, global::TUnit.Mocks.MockEngine<global::IFoo> engine)
+            : base(mockObject, engine) { }
+
+        event global::System.EventHandler<string?> global::IFoo.Something { add => Object.Something += value; remove => Object.Something -= value; }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+    public readonly struct IFoo_MockEvents
+    {
+        internal readonly global::TUnit.Mocks.MockEngine<global::IFoo> Engine;
+
+        internal IFoo_MockEvents(global::TUnit.Mocks.MockEngine<global::IFoo> engine) => Engine = engine;
+    }
+
+    public static class IFoo_MockEventsExtensions
+    {
+        extension(global::TUnit.Mocks.Mock<global::IFoo> mock)
+        {
+            public IFoo_MockEvents Events => new(global::TUnit.Mocks.MockRegistry.GetEngine(mock));
+        }
+
+        extension(IFoo_MockEvents events)
+        {
+            public global::TUnit.Mocks.EventSubscriptionAccessor Something
+                => new(events.Engine, "Something");
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    file sealed class IFooMockImpl : global::IFoo, global::TUnit.Mocks.IRaisable, global::TUnit.Mocks.IMockObject
+    {
+        private readonly global::TUnit.Mocks.MockEngine<global::IFoo> _engine;
+
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        global::TUnit.Mocks.IMock? global::TUnit.Mocks.IMockObject.MockWrapper { get; set; }
+
+        internal IFooMockImpl(global::TUnit.Mocks.MockEngine<global::IFoo> engine)
+        {
+            _engine = engine;
+        }
+
+        private global::System.EventHandler<string?>? _backing_Something;
+
+        public event global::System.EventHandler<string?>? Something
+        {
+            add { _backing_Something += value; _engine.RecordEventSubscription("Something", true); }
+            remove { _backing_Something -= value; _engine.RecordEventSubscription("Something", false); }
+        }
+
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        internal void Raise_Something(string? e)
+        {
+            _backing_Something?.Invoke(this, e);
+        }
+
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        public void RaiseEvent(string eventName, object? args)
+        {
+            switch (eventName)
+            {
+                case "Something":
+                    {
+                        Raise_Something((string?)args!);
+                        break;
+                    }
+                default:
+                    throw new global::System.InvalidOperationException($"No event named '{eventName}' exists on this mock.");
+            }
+        }
+    }
+
+    file static class IFooMockFactory
+    {
+        [global::System.Runtime.CompilerServices.ModuleInitializer]
+        internal static void Register()
+        {
+            global::TUnit.Mocks.MockRegistry.RegisterFactory<global::IFoo>(Create);
+        }
+
+        internal static global::TUnit.Mocks.Mock<global::IFoo> Create(global::TUnit.Mocks.MockBehavior behavior, object[] constructorArgs)
+        {
+            if (constructorArgs.Length > 0) throw new global::System.ArgumentException($"Interface mock 'global::IFoo' does not support constructor arguments, but {constructorArgs.Length} were provided.");
+            var engine = new global::TUnit.Mocks.MockEngine<global::IFoo>(behavior);
+            var impl = new IFooMockImpl(engine);
+            engine.Raisable = impl;
+            var mock = new IFooMock(impl, engine);
+            return mock;
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    public static class IFoo_MockMemberExtensions
+    {
+        public static void RaiseSomething(this global::TUnit.Mocks.Mock<global::IFoo> mock, string? e)
+        {
+            ((global::TUnit.Mocks.IRaisable)global::TUnit.Mocks.MockRegistry.GetEngine(mock).Raisable!).RaiseEvent("Something", (object?)e);
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks
+{
+    public static class IFoo_MockStaticExtension
+    {
+        extension(global::IFoo)
+        {
+            public static global::TUnit.Mocks.Generated.IFooMock Mock(global::TUnit.Mocks.MockBehavior behavior = global::TUnit.Mocks.MockBehavior.Loose)
+            {
+                return (global::TUnit.Mocks.Generated.IFooMock)global::TUnit.Mocks.Mock.Of<global::IFoo>(behavior);
+            }
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated;
\ No newline at end of file
diff --git a/TUnit.Mocks.Tests/Issue5425Tests.cs b/TUnit.Mocks.Tests/Issue5425Tests.cs
new file mode 100644
index 0000000000..6dc35e9b8e
--- /dev/null
+++ b/TUnit.Mocks.Tests/Issue5425Tests.cs
@@ -0,0 +1,65 @@
+#nullable enable
+
+// Promote the nullability warnings that #5424 / #5425 produced into errors so any
+// regression in the source generator (where the generated mock loses nullability
+// information from the source event) breaks this project's build instead of
+// silently emitting warnings.
+//
+//   CS8604 — Possible null reference argument          (#5425)
+//   CS8612 — Nullability mismatch in implemented type  (#5424)
+//   CS8613 — Nullability mismatch in return type
+//   CS8614 — Nullability mismatch in parameter type
+//   CS8615 — Nullability mismatch in implemented member
+#pragma warning error CS8604
+#pragma warning error CS8612
+#pragma warning error CS8613
+#pragma warning error CS8614
+#pragma warning error CS8615
+
+namespace TUnit.Mocks.Tests;
+
+// Compile-time regression coverage for:
+//   - https://github.com/thomhurst/TUnit/issues/5424 — nullable delegate type
+//   - https://github.com/thomhurst/TUnit/issues/5425 — nullable generic type argument
+//
+// If the source generator stops preserving nullability on event handler types,
+// the generated mock will mismatch these source declarations and this file will
+// fail to compile.
+public class Issue5425Tests
+{
+    public interface IWithNullableTypeArg
+    {
+        event EventHandler<string?> Something;
+    }
+
+    public interface IWithNullableEvent
+    {
+        event EventHandler<string>? Something;
+    }
+
+    public interface IWithBothNullable
+    {
+        event EventHandler<string?>? Something;
+    }
+
+    [Test]
+    public async Task Can_Mock_Event_With_Nullable_Type_Argument()
+    {
+        var mock = Mock.Of<IWithNullableTypeArg>();
+        await Assert.That(mock).IsNotNull();
+    }
+
+    [Test]
+    public async Task Can_Mock_Nullable_Event()
+    {
+        var mock = Mock.Of<IWithNullableEvent>();
+        await Assert.That(mock).IsNotNull();
+    }
+
+    [Test]
+    public async Task Can_Mock_Nullable_Event_With_Nullable_Type_Argument()
+    {
+        var mock = Mock.Of<IWithBothNullable>();
+        await Assert.That(mock).IsNotNull();
+    }
+}