diff --git a/src/Http/Http.Extensions/src/RequestDelegateFactory.cs b/src/Http/Http.Extensions/src/RequestDelegateFactory.cs
index c51e7aae5a36..9b0c6d21bdf8 100644
--- a/src/Http/Http.Extensions/src/RequestDelegateFactory.cs
+++ b/src/Http/Http.Extensions/src/RequestDelegateFactory.cs
@@ -1986,6 +1986,15 @@ private static Expression CreateDefaultValueExpression(object? defaultValue, Typ
     {
         if (defaultValue is null)
         {
+            // For non-nullable value types (e.g., Guid, DateTime, TimeSpan), reflection reports
+            // DefaultValue == null when the parameter default is `= default`. Using
+            // Expression.Constant(null, valueType) would throw an ArgumentException because null
+            // is not valid for a non-nullable value type. Use Expression.Default instead.
+            if (parameterType.IsValueType && Nullable.GetUnderlyingType(parameterType) is null)
+            {
+                return Expression.Default(parameterType);
+            }
+
             return Expression.Constant(null, parameterType);
         }
 
diff --git a/src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.QueryParameters.cs b/src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.QueryParameters.cs
index 41dde7279e17..414dee57e5b1 100644
--- a/src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.QueryParameters.cs
+++ b/src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.QueryParameters.cs
@@ -139,6 +139,52 @@ public async Task MapAction_ExplicitParsableParameter_StringReturn()
         await VerifyResponseBodyAsync(httpContext, "10");
     }
 
+    [Theory]
+    [InlineData(null, "Id: 00000000-0000-0000-0000-000000000000")]
+    [InlineData("a0e1f2b3-c4d5-4e6f-7a8b-9c0d1e2f3a4b", "Id: a0e1f2b3-c4d5-4e6f-7a8b-9c0d1e2f3a4b")]
+    public async Task CanSetGuidParamAsOptionalWithDefaultValue(string queryValue, string expectedResponse)
+    {
+        var (_, compilation) = await RunGeneratorAsync("""
+app.MapGet("/", ([FromQuery] Guid id = default) => $"Id: {id}");
+""");
+        var endpoint = GetEndpointFromCompilation(compilation);
+
+        var httpContext = CreateHttpContext();
+        if (queryValue is not null)
+        {
+            httpContext.Request.Query = new QueryCollection(new Dictionary<string, StringValues>
+            {
+                ["id"] = queryValue
+            });
+        }
+
+        await endpoint.RequestDelegate(httpContext);
+        await VerifyResponseBodyAsync(httpContext, expectedResponse);
+    }
+
+    [Theory]
+    [InlineData(null, "Date: 0001-01-01T00:00:00.0000000")]
+    [InlineData("2024-01-15", "Date: 2024-01-15T00:00:00.0000000")]
+    public async Task CanSetDateTimeParamAsOptionalWithDefaultValue(string queryValue, string expectedResponse)
+    {
+        var (_, compilation) = await RunGeneratorAsync("""
+app.MapGet("/", ([FromQuery] DateTime date = default) => $"Date: {date:O}");
+""");
+        var endpoint = GetEndpointFromCompilation(compilation);
+
+        var httpContext = CreateHttpContext();
+        if (queryValue is not null)
+        {
+            httpContext.Request.Query = new QueryCollection(new Dictionary<string, StringValues>
+            {
+                ["date"] = queryValue
+            });
+        }
+
+        await endpoint.RequestDelegate(httpContext);
+        await VerifyResponseBodyAsync(httpContext, expectedResponse);
+    }
+
     public static object[][] MapAction_ExplicitQueryParam_NameTest_Data
     {
         get