Update Slk.php

Author: oleibman

src/PhpSpreadsheet/Reader/Slk.php

@@ -404,7 +404,7 @@ private function addWidth(Spreadsheet $spreadsheet, string $columnWidth, string
                 $endCol = Coordinate::stringFromColumnIndex((int) $endCol);
                 $spreadsheet->getActiveSheet()->getColumnDimension($startCol)->setWidth((float) $columnWidth);
                 do {
-                    $spreadsheet->getActiveSheet()->getColumnDimension(++$startCol)->setWidth((float) $columnWidth);
+                    $spreadsheet->getActiveSheet()->getColumnDimension((string) ++$startCol)->setWidth((float) $columnWidth);
                 } while ($startCol !== $endCol);
             }
         }