diff --git a/app/services/rules_evaluator.rb b/app/services/rules_evaluator.rb
index 99b5a08..e76019a 100644
--- a/app/services/rules_evaluator.rb
+++ b/app/services/rules_evaluator.rb
@@ -93,7 +93,7 @@ class RulesEvaluator
         when "OR"
           (state or result)
         when "XOR"
-          (state or result) and not (rules_state and result)
+          (state or result) and !(rules_state and result)
         else
           raise ArgumentError, "Unrecognized logic '#{rule_set.operator}'"
       end