diff --git a/docs/content/docs/malformed-input/long-method.md b/docs/content/docs/malformed-input/long-method.md
index 9fdf36e..72f93e1 100644
--- a/docs/content/docs/malformed-input/long-method.md
+++ b/docs/content/docs/malformed-input/long-method.md
@@ -8,7 +8,7 @@ weight: 5
 |---|---|
 | **Test ID** | `MAL-LONG-METHOD` |
 | **Category** | Malformed Input |
-| **Expected** | `400` or close |
+| **Expected** | `400`/`501` or close |
 
 ## What it sends
 
diff --git a/src/Http11Probe/TestCases/Suites/MalformedInputSuite.cs b/src/Http11Probe/TestCases/Suites/MalformedInputSuite.cs
index 7a05595..04b34da 100644
--- a/src/Http11Probe/TestCases/Suites/MalformedInputSuite.cs
+++ b/src/Http11Probe/TestCases/Suites/MalformedInputSuite.cs
@@ -231,12 +231,13 @@ public static IEnumerable<TestCase> GetTestCases()
             },
             Expected = new ExpectedBehavior
             {
-                Description = "400 or close",
+                Description = "400, 501 or close",
                 CustomValidator = (response, state) =>
                 {
                     if (response is null)
                         return state == ConnectionState.ClosedByServer ? TestVerdict.Pass : TestVerdict.Fail;
-                    return response.StatusCode == 400
+                    // RFC 9112 §3: a method longer than any the server implements SHOULD get 501.
+                    return response.StatusCode is 400 or 501
                         ? TestVerdict.Pass
                         : TestVerdict.Fail;
                 }