diff --git a/CodeBlocker.Test/TemplateTests.cs b/CodeBlocker.Test/TemplateTests.cs
index 3fd8237..384105a 100644
--- a/CodeBlocker.Test/TemplateTests.cs
+++ b/CodeBlocker.Test/TemplateTests.cs
@@ -530,6 +530,48 @@ public void AnExpressionBodyOfOnlyBlankLinesIsRejectedBeforeAnythingIsWritten()
 		Assert.AreEqual("int a;\n", codeBlocker.ToString());
 	}
 
+	[TestMethod]
+	public void AnExpressionBodyWithADefaultValueIsRejectedBeforeAnythingIsWritten()
+	{
+		// It used to render "int P => x;" and drop the "= 5" without a word.
+		PropertyTemplate property = new()
+		{
+			Type = "int",
+			Name = "P",
+			DefaultValue = "5",
+			ExpressionBodyFactory = codeBlocker => codeBlocker.Write("x"),
+		};
+
+		using CodeBlocker codeBlocker = CodeBlocker.Create(CodeBlocker.DefaultIndentString, NewLines.Lf);
+		codeBlocker.WriteLine("int a;");
+
+		InvalidOperationException exception =
+			Assert.ThrowsExactly<InvalidOperationException>(() => property.WriteTo(codeBlocker));
+		Assert.Contains("'P'", exception.Message);
+		Assert.AreEqual("int a;\n", codeBlocker.ToString());
+	}
+
+	[TestMethod]
+	public void AClassWithAnExpressionBodiedPropertyThatHasADefaultValueWritesNothing()
+	{
+		ClassTemplate type = new() { Name = "C" };
+		type.Members.Add(new FieldTemplate { Name = "a", Type = "int" });
+		type.Members.Add(new PropertyTemplate
+		{
+			Type = "int",
+			Name = "P",
+			DefaultValue = "5",
+			ExpressionBodyFactory = codeBlocker => codeBlocker.Write("x"),
+		});
+
+		using CodeBlocker codeBlocker = CodeBlocker.Create(CodeBlocker.DefaultIndentString, NewLines.Lf);
+
+		InvalidOperationException exception =
+			Assert.ThrowsExactly<InvalidOperationException>(() => codeBlocker.AddClass(type));
+		Assert.Contains("'P'", exception.Message);
+		Assert.AreEqual(string.Empty, codeBlocker.ToString());
+	}
+
 	private static string RenderExpressionBody(Action<CodeBlocker> expressionFactory) =>
 		Render(new PropertyTemplate { Type = "int", Name = "X", Keywords = { "public" }, ExpressionBodyFactory = expressionFactory });
 
diff --git a/CodeBlocker/Templates/PropertyTemplate.cs b/CodeBlocker/Templates/PropertyTemplate.cs
index 471ec0a..cca793d 100644
--- a/CodeBlocker/Templates/PropertyTemplate.cs
+++ b/CodeBlocker/Templates/PropertyTemplate.cs
@@ -48,7 +48,8 @@ public class PropertyTemplate : MemberTemplate
 	/// <exception cref="ArgumentNullException"><paramref name="codeBlocker"/> is <see langword="null"/>.</exception>
 	/// <exception cref="InvalidOperationException">
 	/// <see cref="Getter"/>, <see cref="Setter"/> and <see cref="ExpressionBodyFactory"/> are all
-	/// <see langword="null"/>; or, with no expression body, an accessor's kind is unknown, or is
+	/// <see langword="null"/>; or <see cref="ExpressionBodyFactory"/> is set together with a
+	/// <see cref="TemplateBase.DefaultValue"/>; or, with no expression body, an accessor's kind is unknown, or is
 	/// <see cref="AccessorKind.Expression"/> with no <see cref="AccessorTemplate.BodyFactory"/>; or
 	/// the expression body, or an expression accessor's body, writes no expression. Nothing is
 	/// written in any of these cases.
@@ -98,6 +99,16 @@ internal override void Validate()
 				"Setter, or ExpressionBodyFactory.");
 		}
 
+		if (ExpressionBodyFactory is not null && HasDefaultValue)
+		{
+			// C# has no initialiser on an expression-bodied property, so writing either one would
+			// silently drop the other.
+			throw new InvalidOperationException(
+				$"Property '{Name}' has both an expression body and a default value. An " +
+				"expression-bodied property cannot have an initialiser; clear ExpressionBodyFactory " +
+				"or DefaultValue.");
+		}
+
 		if (ExpressionBodyFactory is null)
 		{
 			// Likewise checked up front: the accessor list is only reached after "Type Name {".