diff --git a/src/libraries/System.Memory/tests/Base64/Base64TestHelper.cs b/src/libraries/System.Memory/tests/Base64/Base64TestHelper.cs
index 417f1994e020e..7715f6b5d4bdf 100644
--- a/src/libraries/System.Memory/tests/Base64/Base64TestHelper.cs
+++ b/src/libraries/System.Memory/tests/Base64/Base64TestHelper.cs
@@ -72,7 +72,7 @@ internal static void InitializeDecodableBytes(Span<byte> bytes, int seed = 100)
             var rnd = new Random(seed);
             for (int i = 0; i < bytes.Length; i++)
             {
-                int index = (byte)rnd.Next(0, s_encodingMap.Length - 1);    // Do not pick '='
+                int index = (byte)rnd.Next(0, s_encodingMap.Length);
                 bytes[i] = s_encodingMap[index];
             }
         }