diff --git a/src/Libraries/Microsoft.Extensions.AI/Functions/AIFunctionFactory.cs b/src/Libraries/Microsoft.Extensions.AI/Functions/AIFunctionFactory.cs
index 4d16ac6ae6b..f81ee89fb6d 100644
--- a/src/Libraries/Microsoft.Extensions.AI/Functions/AIFunctionFactory.cs
+++ b/src/Libraries/Microsoft.Extensions.AI/Functions/AIFunctionFactory.cs
@@ -374,15 +374,14 @@ static bool IsAsyncMethod(MethodInfo method)
                     }
                 }
 
-                // There was no argument for the parameter in the dictionary.
-                // Does it have a default value?
-                if (parameter.HasDefaultValue)
+                // If the parameter is required and there's no argument specified for it, throw.
+                if (!parameter.HasDefaultValue)
                 {
-                    return parameter.DefaultValue;
+                    Throw.ArgumentException(nameof(arguments), $"Missing required parameter '{parameter.Name}' for method '{parameter.Member.Name}'.");
                 }
 
-                // Leave it empty.
-                return null;
+                // Otherwise, use the optional parameter's default value.
+                return parameter.DefaultValue;
             };
         }
 
diff --git a/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs b/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs
index 198627fb14e..8d069034e15 100644
--- a/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs
@@ -78,7 +78,7 @@ public async Task SupportsMultipleFunctionCallsPerRequestAsync(bool concurrentIn
         {
             Tools =
             [
-                AIFunctionFactory.Create((int i) => "Result 1", "Func1"),
+                AIFunctionFactory.Create((int? i = 42) => "Result 1", "Func1"),
                 AIFunctionFactory.Create((int i) => $"Result 2: {i}", "Func2"),
             ]
         };
diff --git a/test/Libraries/Microsoft.Extensions.AI.Tests/Functions/AIFunctionFactoryTest.cs b/test/Libraries/Microsoft.Extensions.AI.Tests/Functions/AIFunctionFactoryTest.cs
index 25f94ac3669..dc104ea6be6 100644
--- a/test/Libraries/Microsoft.Extensions.AI.Tests/Functions/AIFunctionFactoryTest.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.Tests/Functions/AIFunctionFactoryTest.cs
@@ -47,6 +47,24 @@ public async Task Parameters_DefaultValuesAreUsedButOverridable_Async()
         AssertExtensions.EqualFunctionCallResults("hello hello", await func.InvokeAsync([new KeyValuePair<string, object?>("a", "hello")]));
     }
 
+    [Fact]
+    public async Task Parameters_MissingRequiredParametersFail_Async()
+    {
+        AIFunction[] funcs =
+        [
+            AIFunctionFactory.Create((string theParam) => theParam + " " + theParam),
+            AIFunctionFactory.Create((string? theParam) => theParam + " " + theParam),
+            AIFunctionFactory.Create((int theParam) => theParam * 2),
+            AIFunctionFactory.Create((int? theParam) => theParam * 2),
+        ];
+
+        foreach (AIFunction f in funcs)
+        {
+            Exception e = await Assert.ThrowsAsync<ArgumentException>(() => f.InvokeAsync());
+            Assert.Contains("'theParam'", e.Message);
+        }
+    }
+
     [Fact]
     public async Task Parameters_MappedByType_Async()
     {