diff --git a/src/PhpSpreadsheet/Chart/DataSeries.php b/src/PhpSpreadsheet/Chart/DataSeries.php
index 2d64bfbfb..054f088f0 100644
--- a/src/PhpSpreadsheet/Chart/DataSeries.php
+++ b/src/PhpSpreadsheet/Chart/DataSeries.php
@@ -107,14 +107,13 @@ class DataSeries
     /**
      * Create a new DataSeries.
      *
-     * @param null|mixed $plotType
      * @param int[] $plotOrder
      * @param DataSeriesValues[] $plotLabel
      * @param DataSeriesValues[] $plotCategory
      * @param DataSeriesValues[] $plotValues
      */
     public function __construct(
-        $plotType = null,
+        null|string $plotType = null,
         null|string $plotGrouping = null,
         array $plotOrder = [],
         array $plotLabel = [],
diff --git a/src/PhpSpreadsheet/Document/Properties.php b/src/PhpSpreadsheet/Document/Properties.php
index 1459458f8..24a5065cd 100644
--- a/src/PhpSpreadsheet/Document/Properties.php
+++ b/src/PhpSpreadsheet/Document/Properties.php
@@ -394,12 +394,7 @@ class Properties
     /**
      * Set a Custom Property.
      *
-     * @param string $propertyType
-     *   'i' : Integer
-     *   'f' : Floating Point
-     *   's' : String
-     *   'd' : Date/Time
-     *   'b' : Boolean
+     * @param ?string $propertyType see `self::VALID_PROPERTY_TYPE_LIST`
      *
      * @return $this
      */