diff --git a/src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/FrameHeaders.Generated.cs b/src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/FrameHeaders.Generated.cs
index d72bc25e78..39f032ea07 100644
--- a/src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/FrameHeaders.Generated.cs
+++ b/src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/FrameHeaders.Generated.cs
@@ -4197,7 +4197,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
         
         public unsafe void Append(byte[] keyBytes, int keyOffset, int keyLength, string value)
         {
-            var key = new string('\0', keyLength);
+            string key;
             fixed (byte* ptr = &keyBytes[keyOffset])
             {
                 var pUB = ptr;
@@ -4899,6 +4899,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
                         break;
                 }
 
+                key = new string('\0', keyLength);
                 fixed(char *keyBuffer = key)
                 {
                     if (!AsciiUtilities.TryGetAsciiString(ptr, keyBuffer, keyLength))
diff --git a/tools/Microsoft.AspNetCore.Server.Kestrel.GeneratedCode/KnownHeaders.cs b/tools/Microsoft.AspNetCore.Server.Kestrel.GeneratedCode/KnownHeaders.cs
index 53dff0dd53..ed126ec58b 100644
--- a/tools/Microsoft.AspNetCore.Server.Kestrel.GeneratedCode/KnownHeaders.cs
+++ b/tools/Microsoft.AspNetCore.Server.Kestrel.GeneratedCode/KnownHeaders.cs
@@ -417,7 +417,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
         {(loop.ClassName == "FrameRequestHeaders" ? $@"
         public unsafe void Append(byte[] keyBytes, int keyOffset, int keyLength, string value)
         {{
-            var key = new string('\0', keyLength);
+            string key;
             fixed (byte* ptr = &keyBytes[keyOffset])
             {{
                 var pUB = ptr;
@@ -446,6 +446,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
                         break;
                 ")}}}
 
+                key = new string('\0', keyLength);
                 fixed(char *keyBuffer = key)
                 {{
                     if (!AsciiUtilities.TryGetAsciiString(ptr, keyBuffer, keyLength))