diff --git a/processing/src/main/java/edu/harvard/hms/dbmi/avillach/hpds/processing/timeseries/TimeSeriesConversionService.java b/processing/src/main/java/edu/harvard/hms/dbmi/avillach/hpds/processing/timeseries/TimeSeriesConversionService.java
index 2a9f4ffe..816a66fa 100644
--- a/processing/src/main/java/edu/harvard/hms/dbmi/avillach/hpds/processing/timeseries/TimeSeriesConversionService.java
+++ b/processing/src/main/java/edu/harvard/hms/dbmi/avillach/hpds/processing/timeseries/TimeSeriesConversionService.java
@@ -9,6 +9,9 @@
 public class TimeSeriesConversionService {
 
     public String toISOString(Long unixTimeStamp) {
+        if (unixTimeStamp == null) {
+            return "";
+        }
         Instant instant = Instant.ofEpochMilli(unixTimeStamp);
         return DateTimeFormatter.ISO_INSTANT.format(instant);
     }