diff --git a/src/SharpCompress/Compressors/Rar/UnpackV1/Unpack20.cs b/src/SharpCompress/Compressors/Rar/UnpackV1/Unpack20.cs
index 809aaf60e..19af8c857 100644
--- a/src/SharpCompress/Compressors/Rar/UnpackV1/Unpack20.cs
+++ b/src/SharpCompress/Compressors/Rar/UnpackV1/Unpack20.cs
@@ -369,7 +369,7 @@ private void CopyString20(int Length, int Distance)
         destUnpSize -= Length;
 
         var DestPtr = unpPtr - Distance;
-        if (DestPtr < PackDef.MAXWINSIZE - 300 && unpPtr < PackDef.MAXWINSIZE - 300)
+        if (DestPtr >= 0 && DestPtr < PackDef.MAXWINSIZE - 300 && unpPtr < PackDef.MAXWINSIZE - 300)
         {
             window[unpPtr++] = window[DestPtr++];
             window[unpPtr++] = window[DestPtr++];