diff --git a/src/libraries/System.Net.HttpListener/src/System/Net/Managed/HttpListenerRequest.Managed.cs b/src/libraries/System.Net.HttpListener/src/System/Net/Managed/HttpListenerRequest.Managed.cs
index 098782eec22151..fba279bdc46489 100644
--- a/src/libraries/System.Net.HttpListener/src/System/Net/Managed/HttpListenerRequest.Managed.cs
+++ b/src/libraries/System.Net.HttpListener/src/System/Net/Managed/HttpListenerRequest.Managed.cs
@@ -219,15 +219,9 @@ internal void AddHeader(string header)
             string val = header.AsSpan(colon + 1).Trim().ToString();
             if (name.Equals("content-length", StringComparison.OrdinalIgnoreCase))
             {
-                // To match Windows behavior:
-                // Content lengths >= 0 and <= long.MaxValue are accepted as is.
-                // Content lengths > long.MaxValue and <= ulong.MaxValue are treated as 0.
-                // Content lengths < 0 cause the requests to fail.
-                // Other input is a failure, too.
-                long parsedContentLength =
-                    ulong.TryParse(val, out ulong parsedUlongContentLength) ? (parsedUlongContentLength <= long.MaxValue ? (long)parsedUlongContentLength : 0) :
-                    long.Parse(val);
-                if (parsedContentLength < 0 || (_clSet && parsedContentLength != _contentLength))
+                // Match the Windows parser shape: strict decimal parsing, and reject on parse failure.
+                bool success = long.TryParse(val, NumberStyles.None, CultureInfo.InvariantCulture.NumberFormat, out long parsedContentLength);
+                if (!success || (_clSet && parsedContentLength != _contentLength))
                 {
                     _context.ErrorMessage = "Invalid Content-Length.";
                 }
diff --git a/src/libraries/System.Net.HttpListener/tests/HttpListenerRequestTests.cs b/src/libraries/System.Net.HttpListener/tests/HttpListenerRequestTests.cs
index 9e32572bb8322f..b6f41a169e72fa 100644
--- a/src/libraries/System.Net.HttpListener/tests/HttpListenerRequestTests.cs
+++ b/src/libraries/System.Net.HttpListener/tests/HttpListenerRequestTests.cs
@@ -126,8 +126,6 @@ public async Task ContentEncoding_NoBody_ReturnsDefault()
 
         [Theory]
         [InlineData("POST", "Content-Length: 9223372036854775807", 9223372036854775807, true)] // long.MaxValue
-        [InlineData("POST", "Content-Length: 9223372036854775808", 0, false)] // long.MaxValue + 1
-        [InlineData("POST", "Content-Length: 18446744073709551615 ", 0, false)] // ulong.MaxValue
         [InlineData("POST", "Content-Length: 0", 0, false)]
         [InlineData("PUT", "Content-Length: 0", 0, false)]
         [InlineData("PUT", "Content-Length: 1", 1, true)]
diff --git a/src/libraries/System.Net.HttpListener/tests/InvalidClientRequestTests.cs b/src/libraries/System.Net.HttpListener/tests/InvalidClientRequestTests.cs
index 67e7c187225fba..784987870827d5 100644
--- a/src/libraries/System.Net.HttpListener/tests/InvalidClientRequestTests.cs
+++ b/src/libraries/System.Net.HttpListener/tests/InvalidClientRequestTests.cs
@@ -74,6 +74,8 @@ public static IEnumerable<object[]> InvalidRequest_TestData()
             yield return new object[] { "GET {path} HTTP/1.1", null, new string[] { "Content-Length: -9223372036854775809" }, "\r\n", "Bad Request" };
 
             yield return new object[] { "GET {path} HTTP/1.1", null, new string[] { "Content-Length: 1", "Content-Length: 2" }, "\r\n", "Bad Request" };
+            yield return new object[] { "POST {path} HTTP/1.1", null, new string[] { "Content-Length: 9223372036854775808" }, "\r\n", "Bad Request" }; // long.MaxValue + 1
+            yield return new object[] { "POST {path} HTTP/1.1", null, new string[] { "Content-Length: 18446744073709551615" }, "\r\n", "Bad Request" }; // ulong.MaxValue
 
             yield return new object[] { "GET {path} HTTP/1.1", null, new string[] { "Transfer-Encoding: garbage" }, "\r\n", "Not Implemented" };
             yield return new object[] { "POST {path} HTTP/1.1", null, new string[] { "Transfer-Encoding: garbage" }, "\r\n", "Not Implemented" };