diff --git a/app/com/linkedin/drelephant/mapreduce/MapReduceMetricsAggregator.java b/app/com/linkedin/drelephant/mapreduce/MapReduceMetricsAggregator.java
index 5d00dead6..ca29ad158 100644
--- a/app/com/linkedin/drelephant/mapreduce/MapReduceMetricsAggregator.java
+++ b/app/com/linkedin/drelephant/mapreduce/MapReduceMetricsAggregator.java
@@ -57,6 +57,12 @@ public void aggregate(HadoopApplicationData hadoopData) {
 
     long reduceIdealStartTime = mapTasks.getNthPercentileFinishTime(reduceTaskSlowStartPercentage);
 
+    // Mappers list is empty
+    if(reduceIdealStartTime == -1) {
+      // ideal start time for reducer is infinite since it cannot start
+      reduceIdealStartTime = Long.MAX_VALUE;
+    }
+
     reduceTasks = new TaskLevelAggregatedMetrics(data.getReducerData(), reduceTaskContainerSize, reduceIdealStartTime);
 
     _hadoopAggregatedData.setResourceUsed(mapTasks.getResourceUsed() + reduceTasks.getResourceUsed());
diff --git a/app/com/linkedin/drelephant/mapreduce/TaskLevelAggregatedMetrics.java b/app/com/linkedin/drelephant/mapreduce/TaskLevelAggregatedMetrics.java
index 2a18df679..f0a525943 100644
--- a/app/com/linkedin/drelephant/mapreduce/TaskLevelAggregatedMetrics.java
+++ b/app/com/linkedin/drelephant/mapreduce/TaskLevelAggregatedMetrics.java
@@ -46,6 +46,9 @@ public class TaskLevelAggregatedMetrics {
    */
   public long getNthPercentileFinishTime(int percentile)
   {
+    if(finishTimes == null || finishTimes.size() == 0 ) {
+      return -1;
+    }
     return Statistics.percentile(finishTimes, percentile);
   }
 
@@ -98,7 +101,7 @@ private void compute(MapReduceTaskData[] taskDatas, long containerSize, long ide
     long taskDurationMax = 0;
 
     // if there are zero tasks, then nothing to compute.
-    if(taskDatas.length == 0) {
+    if(taskDatas == null || taskDatas.length == 0) {
       return;
     }
 
@@ -133,6 +136,11 @@ private void compute(MapReduceTaskData[] taskDatas, long containerSize, long ide
     // Compute the delay in starting the task.
     _delay = taskFinishTimeMax - (idealStartTime + taskDurationMax);
 
+    // invalid delay
+    if(_delay < 0) {
+      _delay = 0;
+    }
+
     // wastedResources
     long wastedMemory = containerSize -  (long) (peakMemoryNeed * MEMORY_BUFFER); // give a 50% buffer
     if(wastedMemory > 0) {
diff --git a/public/images/runtime.png b/public/images/runtime.png
index 68df97405..6f98430d9 100644
Binary files a/public/images/runtime.png and b/public/images/runtime.png differ
diff --git a/public/images/usedmemory.png b/public/images/usedmemory.png
index e637efc76..c55a084a6 100644
Binary files a/public/images/usedmemory.png and b/public/images/usedmemory.png differ
diff --git a/public/images/waittime.png b/public/images/waittime.png
index cfd494c3c..fb862f961 100644
Binary files a/public/images/waittime.png and b/public/images/waittime.png differ
diff --git a/public/images/wastedmemory.png b/public/images/wastedmemory.png
index 37e68b534..627bf8d9c 100644
Binary files a/public/images/wastedmemory.png and b/public/images/wastedmemory.png differ
diff --git a/test/com/linkedin/drelephant/mapreduce/TestTaskLevelAggregatedMetrics.java b/test/com/linkedin/drelephant/mapreduce/TestTaskLevelAggregatedMetrics.java
index 3163d732f..f5dd528c7 100644
--- a/test/com/linkedin/drelephant/mapreduce/TestTaskLevelAggregatedMetrics.java
+++ b/test/com/linkedin/drelephant/mapreduce/TestTaskLevelAggregatedMetrics.java
@@ -32,6 +32,14 @@ public void testZeroTasks() {
         Assert.assertEquals(taskMetrics.getResourceWasted(), 0);
     }
 
+    @Test
+    public void testNullTaskArray() {
+        TaskLevelAggregatedMetrics taskMetrics = new TaskLevelAggregatedMetrics(null, 0, 0);
+        Assert.assertEquals(taskMetrics.getDelay(), 0);
+        Assert.assertEquals(taskMetrics.getResourceUsed(), 0);
+        Assert.assertEquals(taskMetrics.getResourceWasted(), 0);
+    }
+
     @Test
     public void testTaskLevelData() {
         MapReduceTaskData taskData[] = new MapReduceTaskData[2];