diff --git a/src/Kestrel.Core/Internal/Http/ReasonPhrases.cs b/src/Kestrel.Core/Internal/Http/ReasonPhrases.cs
index 36eaaacf87..d372113bda 100644
--- a/src/Kestrel.Core/Internal/Http/ReasonPhrases.cs
+++ b/src/Kestrel.Core/Internal/Http/ReasonPhrases.cs
@@ -222,9 +222,11 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http
 
                     default:
                         var predefinedReasonPhrase = WebUtilities.ReasonPhrases.GetReasonPhrase(statusCode);
+                        // https://tools.ietf.org/html/rfc7230#section-3.1.2 requires trailing whitespace regardless of reason phrase
+                        var formattedStatusCode = statusCode.ToString(CultureInfo.InvariantCulture) + " ";
                         return string.IsNullOrEmpty(predefinedReasonPhrase)
-                            ? Encoding.ASCII.GetBytes(statusCode.ToString(CultureInfo.InvariantCulture))
-                            : Encoding.ASCII.GetBytes(statusCode.ToString(CultureInfo.InvariantCulture) + " " + predefinedReasonPhrase);
+                            ? Encoding.ASCII.GetBytes(formattedStatusCode)
+                            : Encoding.ASCII.GetBytes(formattedStatusCode + predefinedReasonPhrase);
 
                 }
             }
diff --git a/test/Kestrel.Core.Tests/ReasonPhrasesTests.cs b/test/Kestrel.Core.Tests/ReasonPhrasesTests.cs
new file mode 100644
index 0000000000..9a7957735b
--- /dev/null
+++ b/test/Kestrel.Core.Tests/ReasonPhrasesTests.cs
@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Xunit;
+using Microsoft.AspNetCore.Http;
+using System.Text;
+
+namespace Microsoft.AspNetCore.Server.Kestrel.Core.Tests
+{
+    public class ReasonPhraseTests
+    {
+        [Theory]
+        [InlineData(999, "Unknown", "999 Unknown")]
+        [InlineData(999, null, "999 ")]
+        [InlineData(StatusCodes.Status200OK, "OK", "200 OK")]
+        [InlineData(StatusCodes.Status200OK, null, "200 OK")]
+        public void Formatting(int statusCode, string reasonPhrase, string expectedResult)
+        {
+            var bytes = Internal.Http.ReasonPhrases.ToStatusBytes(statusCode, reasonPhrase);
+            Assert.NotNull(bytes);
+            Assert.Equal(expectedResult, Encoding.ASCII.GetString(bytes));
+        }
+    }
+}
\ No newline at end of file