From 3652c9b105ccf15fe73dfb15a6803ae3bba29d6f Mon Sep 17 00:00:00 2001
From: John Karam <168653085+johnkaram02@users.noreply.github.com>
Date: Fri, 23 Oct 2026 20:22:32 +0000
Subject: [PATCH] .Net: Use parameter default value when a null argument
 targets a non-nullable value type

When a KernelArguments value is null and the method parameter is a non-nullable value type with a default value, KernelFunctionFromMethod tried to convert null and threw ArgumentOutOfRangeException. OpenAI strict mode makes optional parameters nullable, so models send null for them and the call fails. Fall back to the parameter's default value in that case.
---
 .../Functions/KernelFunctionFromMethod.cs          |  6 ++++--
 .../Functions/KernelFunctionFromMethodTests1.cs    | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/dotnet/src/SemanticKernel.Core/Functions/KernelFunctionFromMethod.cs b/dotnet/src/SemanticKernel.Core/Functions/KernelFunctionFromMethod.cs
index 9385ae02bcaf..4dc83cb46c7d 100644
--- a/dotnet/src/SemanticKernel.Core/Functions/KernelFunctionFromMethod.cs
+++ b/dotnet/src/SemanticKernel.Core/Functions/KernelFunctionFromMethod.cs
@@ -697,8 +697,10 @@ private static (Func<KernelFunction, Kernel, KernelArguments, CancellationToken,
 
         object? parameterFunc(KernelFunction _, Kernel kernel, KernelArguments arguments, CancellationToken __)
         {
-            // 1. Use the value of the variable if it exists.
-            if (arguments.TryGetValue(name, out object? value))
+            // 1. Use the value of the variable if it exists. A null value can't be assigned to a non-nullable value type,
+            // so skip it if the parameter has a default value, e.g. when the AI model sends null for an optional parameter.
+            if (arguments.TryGetValue(name, out object? value) &&
+                (value is not null || !parameter.HasDefaultValue || !type.IsValueType || Nullable.GetUnderlyingType(type) is not null))
             {
                 return Process(value);
             }
diff --git a/dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromMethodTests1.cs b/dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromMethodTests1.cs
index 2c154029617a..9e234945fcff 100644
--- a/dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromMethodTests1.cs
+++ b/dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromMethodTests1.cs
@@ -989,6 +989,20 @@ public async Task ItShouldMarshalArgumentsOfValueTypeAsync()
         Assert.Null(actual);
     }
 
+    [Fact]
+    public async Task ItShouldUseDefaultValueWhenNullIsPassedForValueTypeParameterWithDefaultValueAsync()
+    {
+        // Arrange
+        object? actual = null;
+        var sut = KernelFunctionFactory.CreateFromMethod((int val = 5) => { actual = val; });
+
+        // Act
+        await sut.InvokeAsync(this._kernel, new() { ["val"] = null });
+
+        // Assert
+        Assert.Equal(5, actual);
+    }
+
     [Fact]
     public async Task ItShouldMarshalArgumentsOfReferenceTypeAsync()
     {