diff --git a/src/System.Text.Encoding.CodePages/src/System/Text/EncodingCharBuffer.cs b/src/System.Text.Encoding.CodePages/src/System/Text/EncodingCharBuffer.cs
index 16c8afe94aeb..bdb19d52059f 100644
--- a/src/System.Text.Encoding.CodePages/src/System/Text/EncodingCharBuffer.cs
+++ b/src/System.Text.Encoding.CodePages/src/System/Text/EncodingCharBuffer.cs
@@ -90,7 +90,7 @@ internal unsafe bool AddChar(char ch1, char ch2, int numBytes)
         [System.Security.SecurityCritical]  // auto-generated
         internal unsafe void AdjustBytes(int count)
         {
-            _bytes += count;
+            _bytes = unchecked(_bytes + count);
         }
 
         internal unsafe bool MoreData