diff --git a/src/libraries/System.Private.Xml/src/System/Xml/Serialization/CodeIdentifier.cs b/src/libraries/System.Private.Xml/src/System/Xml/Serialization/CodeIdentifier.cs
index 89afc4d625d1e9..1938972d79eac0 100644
--- a/src/libraries/System.Private.Xml/src/System/Xml/Serialization/CodeIdentifier.cs
+++ b/src/libraries/System.Private.Xml/src/System/Xml/Serialization/CodeIdentifier.cs
@@ -283,7 +283,7 @@ private static void EscapeKeywords(string identifier, StringBuilder sb)
             {
                 if (separator >= 0)
                 {
-                    sb.Append(originalIdentifier.Substring(separator, 1));
+                    sb.Append(originalIdentifier[separator]);
                 }
                 separator++;
                 separator += names[i].Length;