diff --git a/src/libraries/System.Private.CoreLib/src/System/String.cs b/src/libraries/System.Private.CoreLib/src/System/String.cs
index 9489b7b0b3c06a..9e4916749e9ee6 100644
--- a/src/libraries/System.Private.CoreLib/src/System/String.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/String.cs
@@ -531,7 +531,8 @@ public static bool IsNullOrWhiteSpace([NotNullWhen(false)] string? value)
         internal static string Alloc(int length, out Span<char> resultSpan)
         {
             string result = FastAllocateString(length);
-            resultSpan = new Span<char>(ref result._firstChar, result._stringLength);
+            Debug.Assert(result.Length == length);
+            resultSpan = new Span<char>(ref result._firstChar, length);
             return result;
         }