app/models/cohortly/report.rb in cohortly-0.0.8 vs app/models/cohortly/report.rb in cohortly-0.0.9

@@ -66,10 +66,14 @@
       res
     end
     
     def period_cohorts_from(cohort_key)
       index = period_cohorts.index(cohort_key)
-      period_cohorts[index..-1]
+      if index 
+        period_cohorts[index..-1]  
+      else
+        []
+      end
     end
 
     def report_line(cohort_key)
       line = data.find {|x| x['_id'] == cohort_key}
       return [] unless line