.Net: Use parameter default value when a null argument targets a non-nullable value type

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);
             }

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()
     {