diff --git a/app/models/time_series/trend.rb b/app/models/time_series/trend.rb
index cde8e2c1..70f9077e 100644
--- a/app/models/time_series/trend.rb
+++ b/app/models/time_series/trend.rb
@@ -59,7 +59,7 @@ class TimeSeries::Trend
     attr_reader :series
 
     def values_must_be_of_same_type
-      unless current.class == previous.class || previous.nil?
+      unless current.class == previous.class || [ previous, current ].any?(&:nil?)
         errors.add :current, "must be of the same type as previous"
         errors.add :previous, "must be of the same type as current"
       end