diff --git a/library/Icinga/Chart/Graph/BarGraph.php b/library/Icinga/Chart/Graph/BarGraph.php
index adef42894..33429f5a9 100644
--- a/library/Icinga/Chart/Graph/BarGraph.php
+++ b/library/Icinga/Chart/Graph/BarGraph.php
@@ -67,6 +67,7 @@ class BarGraph extends Styleable implements Drawable
         $this->dataSet = $dataSet;
 
         $this->tooltips = $tooltips;
+        $ts = [];
         foreach ($this->tooltips as $value) {
             $ts[] = $value;
         }
diff --git a/library/Icinga/Chart/Graph/LineGraph.php b/library/Icinga/Chart/Graph/LineGraph.php
index 6954c596f..05f3a310f 100644
--- a/library/Icinga/Chart/Graph/LineGraph.php
+++ b/library/Icinga/Chart/Graph/LineGraph.php
@@ -74,6 +74,7 @@ class LineGraph extends Styleable implements Drawable
         $this->graphs = $graphs;
 
         $this->tooltips = $tooltips;
+        $ts = [];
         foreach ($this->tooltips as $value) {
             $ts[] = $value;
         }