diff --git a/WebApp/src/uk/ac/exeter/QuinCe/web/datasets/SensorOffsets/TimeSeriesPlotData.java b/WebApp/src/uk/ac/exeter/QuinCe/web/datasets/SensorOffsets/TimeSeriesPlotData.java
index 8a57436a2..a7545ad3c 100644
--- a/WebApp/src/uk/ac/exeter/QuinCe/web/datasets/SensorOffsets/TimeSeriesPlotData.java
+++ b/WebApp/src/uk/ac/exeter/QuinCe/web/datasets/SensorOffsets/TimeSeriesPlotData.java
@@ -122,11 +122,13 @@ protected String getArray(SensorOffsets sensorOffsets,
   }
 
   protected Double getFirstSeriesValue(LocalDateTime time) {
-    return data.get(time).getFirst();
+    Tuple tuple = data.get(time);
+    return null == tuple ? Double.NaN : tuple.getFirst();
   }
 
   protected Double getSecondSeriesValue(LocalDateTime time) {
-    return data.get(time).getSecond();
+    Tuple tuple = data.get(time);
+    return null == tuple ? Double.NaN : tuple.getSecond();
   }
 
   /**