--- CPP/7zip/Archive/7z/7zIn.cpp	
+++ CPP/7zip/Archive/7z/7zIn.cpp	
@@ -1097,7 +1097,8 @@ HRESULT CInArchive::ReadAndDecodePackedS
       if (CrcCalc(data, unpackSize) != folders.FolderCRCs.Vals[i])
         ThrowIncorrect();
   }
-  HeadersSize += folders.PackPositions[folders.NumPackStreams];
+  if (folders.PackPositions)
+    HeadersSize += folders.PackPositions[folders.NumPackStreams];
   return S_OK;
 }