diff --git a/Library/Homebrew/rubocops/shared/desc_helper.rb b/Library/Homebrew/rubocops/shared/desc_helper.rb
index 5e1e273c514ce24514478e42adb621682f82d5c2..e780ad4503492865733391cd1a60130bf1a92fe0 100644
--- a/Library/Homebrew/rubocops/shared/desc_helper.rb
+++ b/Library/Homebrew/rubocops/shared/desc_helper.rb
@@ -65,7 +65,7 @@ module RuboCop
         problem "Description shouldn't start with the #{type} name." if regex_match_group(desc, /^#{name_regex}\b/i)
 
         if type == :cask && match = regex_match_group(desc, /\b(macOS|Mac( ?OS( ?X)?)?|OS ?X)\b/i)
-          problem "Description shouldn't contain the platform." unless match[1] == "MAC"
+          problem "Description shouldn't contain the platform." if match[1] != "MAC"
         end
 
         # Check if a full stop is used at the end of a desc (apart from in the case of "etc.").