diff --git a/Algorithm/Clustering/Kmeans.php b/Algorithm/Clustering/Kmeans.php
index f9ef03493..d084b4aed 100644
--- a/Algorithm/Clustering/Kmeans.php
+++ b/Algorithm/Clustering/Kmeans.php
@@ -136,6 +136,7 @@ final class Kmeans
             foreach ($points as $point) {
                 $clusterPoint = $clusterCenters[$point->getGroup()];
 
+                // this should ensure that clusterPoint and therfore the center group is never 0. But this is not true.
                 $clusterPoint->setGroup(
                     $clusterPoint->getGroup() + 1
                 );
diff --git a/Algorithm/Clustering/Point.php b/Algorithm/Clustering/Point.php
index 07c3eb833..b3d868f7b 100644
--- a/Algorithm/Clustering/Point.php
+++ b/Algorithm/Clustering/Point.php
@@ -27,7 +27,7 @@ class Point implements PointInterface
     /**
      * Coordinates of the point
      *
-     * @var   array
+     * @var   array<int, int|float>
      * @sicne 1.0.0
      */
     private array $coordinates = [];
@@ -81,7 +81,7 @@ class Point implements PointInterface
     /**
      * {@inheritdoc}
      */
-    public function setCoordinate($index, $value)
+    public function setCoordinate($index, $value) : void
     {
         $this->coordinates[$index] = $value;
     }
diff --git a/Algorithm/Clustering/PointInterface.php b/Algorithm/Clustering/PointInterface.php
index a14d436ff..02a258d71 100644
--- a/Algorithm/Clustering/PointInterface.php
+++ b/Algorithm/Clustering/PointInterface.php
@@ -39,7 +39,7 @@ interface PointInterface
      *
      * @param mixed $index Index of the coordinate (e.g. 0 = x);
      *
-     * @return array
+     * @return int|float
      *
      * @since 1.0.0
      */
@@ -51,11 +51,11 @@ interface PointInterface
      * @param mixed $index Index of the coordinate (e.g. 0 = x);
      * @param mixed $value Value of the coordinate
      *
-     * @return array
+     * @return void
      *
      * @since 1.0.0
      */
-    public function setCoordinate($index, $value);
+    public function setCoordinate($index, $value) : void;
 
     /**
      * Get group this point belongs to