diff --git a/WebJobBillingData/RecordDataReader.cs b/WebJobBillingData/RecordDataReader.cs
index e4b2ee8..5001b54 100644
--- a/WebJobBillingData/RecordDataReader.cs
+++ b/WebJobBillingData/RecordDataReader.cs
@@ -276,7 +276,7 @@ private static string SerializeDictionary(IDictionary dictionary)
 				writer.WritePropertyName("Name");
 				writer.WriteValue(key.ToString());
 				writer.WritePropertyName("Value");
-				writer.WriteValue(value.ToString());
+				writer.WriteValue((value ?? "").ToString());
 				writer.WriteEndObject();
 			}
 			writer.WriteEndArray();