diff --git a/README.md b/README.md
index efd9530..3fb72f5 100644
--- a/README.md
+++ b/README.md
@@ -255,7 +255,7 @@ land in the test project's compile references and collide with the in-box types
 | `Initialize(IncrementalGeneratorInitializationContext)` | `void` | Finds the declared files and registers the source output. Sealed behaviour; do not reimplement. |
 | `Generate(SourceProductionContext, MetadataSet)` | `void` | Emits this generator's sources. Abstract. |
 | `CreateCodeBlocker()` | `CodeBlocker` | A `CodeBlocker` pinned to LF, so output is byte-identical on every platform. Static. |
-| `WriteFileHeader(CodeBlocker, string?)` | `void` | Writes the copyright line and the `<auto-generated />` marker. A null or empty copyright writes only the marker. Static. |
+| `WriteFileHeader(CodeBlocker, string?)` | `void` | Writes the copyright line, the `<auto-generated />` marker and `#nullable enable` (generated code otherwise compiles with nullable disabled). A null or empty copyright writes no copyright lines. Static. |
 | `WriteSourceFile(CodeBlocker, SourceFileTemplate, string?)` | `void` | Writes a whole source file, header included. Static. |
 
 ### `GeneratorBase<T>`
diff --git a/SourceGeneratorToolkit.Test/GeneratorBaseTests.cs b/SourceGeneratorToolkit.Test/GeneratorBaseTests.cs
index 38ba293..5650de2 100644
--- a/SourceGeneratorToolkit.Test/GeneratorBaseTests.cs
+++ b/SourceGeneratorToolkit.Test/GeneratorBaseTests.cs
@@ -4,6 +4,7 @@ namespace ktsu.SourceGeneratorToolkit.Test;
 
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.IO;
 using System.Linq;
 using Microsoft.CodeAnalysis;
@@ -293,10 +294,50 @@ public void AMultiLineCopyrightIsCommentedOnEveryLine(string lineBreak)
 	{
 		string source = HeaderProbe.Write($"Copyright (c) Example.{lineBreak}{lineBreak}All rights reserved.");
 
-		Assert.AreEqual("// Copyright (c) Example.\n//\n// All rights reserved.\n// <auto-generated />\n\nnamespace Generated { }\n", source);
+		Assert.AreEqual("// Copyright (c) Example.\n//\n// All rights reserved.\n// <auto-generated />\n#nullable enable\n\nnamespace Generated { }\n", source);
 		Assert.AreEqual(0, CSharpSyntaxTree.ParseText(source).GetDiagnostics().Count(), source);
 	}
 
+	[TestMethod]
+	public void TheHeaderEnablesNullableAfterTheGeneratedMarker() =>
+		StringAssert.Contains(HeaderProbe.Write(null), "// <auto-generated />\n#nullable enable\n");
+
+	[TestMethod]
+	public void AnnotatedGeneratedMembersCompileWithoutCS8669()
+	{
+		ImmutableArray<Diagnostic> diagnostics = CompileWithConsumer("int n = t.Description?.Length ?? 0;");
+
+		Assert.IsFalse(diagnostics.Any(d => d.Id == "CS8669"), string.Join("\n", diagnostics));
+	}
+
+	[TestMethod]
+	public void UnannotatedGeneratedMembersAreNotNullOblivious()
+	{
+		ImmutableArray<Diagnostic> diagnostics = CompileWithConsumer("t.Name = null;");
+
+		Assert.IsTrue(diagnostics.Any(d => d.Id == "CS8625"), string.Join("\n", diagnostics));
+	}
+
+	/// <summary>
+	/// Compiles a <see cref="GeneratorBase.WriteSourceFile"/> class holding a <c>string?</c> and a
+	/// <c>string</c> member, next to a consumer statement, the way a nullable-enabled project would.
+	/// </summary>
+	private static ImmutableArray<Diagnostic> CompileWithConsumer(string statement)
+	{
+		string generated = SourceFileProbe.WriteThingWithNullableAndNonNullableMembers();
+		string consumer = $"namespace Consumer {{ public static class User {{ public static void Use(Generated.Thing t) {{ {statement} }} }} }}";
+
+		CSharpCompilation compilation = CSharpCompilation.Create(
+			assemblyName: "NullableConsumer",
+			syntaxTrees: [CSharpSyntaxTree.ParseText(generated, path: "Thing.g.cs"), CSharpSyntaxTree.ParseText(consumer, path: "User.cs")],
+			references: [MetadataReference.CreateFromFile(typeof(object).Assembly.Location)],
+			options: new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary, nullableContextOptions: NullableContextOptions.Enable));
+
+		ImmutableArray<Diagnostic> diagnostics = compilation.GetDiagnostics();
+		Assert.IsFalse(diagnostics.Any(d => d.Severity == DiagnosticSeverity.Error), generated + "\n" + string.Join("\n", diagnostics));
+		return diagnostics;
+	}
+
 	[TestMethod]
 	public void TheGeneratorReusesItsOutputWhenNothingChanged()
 	{
diff --git a/SourceGeneratorToolkit.Test/TestGenerators.cs b/SourceGeneratorToolkit.Test/TestGenerators.cs
index 7afef9c..568b33e 100644
--- a/SourceGeneratorToolkit.Test/TestGenerators.cs
+++ b/SourceGeneratorToolkit.Test/TestGenerators.cs
@@ -340,3 +340,32 @@ internal static string Write(string? copyright)
 		return codeBlocker.ToString();
 	}
 }
+
+/// <summary>
+/// Exposes the protected whole-file writer, so its output can be compiled alongside a consumer.
+/// </summary>
+internal abstract class SourceFileProbe : GeneratorBase
+{
+	/// <summary>
+	/// Writes <c>Generated.Thing</c> with a <c>string?</c> field and a <c>string</c> field.
+	/// </summary>
+	internal static string WriteThingWithNullableAndNonNullableMembers()
+	{
+		SourceFileTemplate file = new() { Namespace = "Generated" };
+		file.Classes.Add(new ClassTemplate
+		{
+			Name = "Thing",
+			Kind = ktsu.CodeBlocker.Templates.TypeKind.Class,
+			Keywords = { CSharpKeywords.Public },
+			Members =
+			{
+				new FieldTemplate { Name = "Description", Type = "string?", Keywords = { CSharpKeywords.Public } },
+				new FieldTemplate { Name = "Name", Type = "string", DefaultValue = "\"\"", Keywords = { CSharpKeywords.Public } },
+			},
+		});
+
+		using CodeBlocker codeBlocker = CreateCodeBlocker();
+		WriteSourceFile(codeBlocker, file, TestDiagnostics.Copyright);
+		return codeBlocker.ToString();
+	}
+}
diff --git a/SourceGeneratorToolkit/GeneratorBase.cs b/SourceGeneratorToolkit/GeneratorBase.cs
index 0ba1891..fa152ad 100644
--- a/SourceGeneratorToolkit/GeneratorBase.cs
+++ b/SourceGeneratorToolkit/GeneratorBase.cs
@@ -128,6 +128,12 @@ protected static CodeBlocker CreateCodeBlocker() =>
 	/// A parameter rather than a hard-coded literal, so a consuming repository can keep the header in
 	/// step with its own file header template from one place, and so nothing about any one
 	/// repository leaks into this package.
+	/// <para>
+	/// The header ends with <c>#nullable enable</c>. Roslyn compiles a file carrying the
+	/// generated-file marker with the nullable context disabled, whatever the consuming project's
+	/// <c>Nullable</c> setting, so without the directive every <c>?</c> annotation in the output
+	/// raises CS8669 and every unannotated reference member is null-oblivious to its callers.
+	/// </para>
 	/// </remarks>
 	/// <exception cref="ArgumentNullException"><paramref name="codeBlocker"/> is <see langword="null"/>.</exception>
 	protected static void WriteFileHeader(CodeBlocker codeBlocker, string? copyright)
@@ -143,6 +149,7 @@ protected static void WriteFileHeader(CodeBlocker codeBlocker, string? copyright
 		}
 
 		codeBlocker.WriteLine("// <auto-generated />");
+		codeBlocker.WriteLine("#nullable enable");
 		codeBlocker.NewLine();
 	}