diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8a24952a..f82de5f0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -32,10 +32,8 @@ planned for 2025-07-01
 ### Fixed
 
 - [fix] Handle spellcheck issues (#3783)
-
-### Fixed
-
 - [calendar] fix fullday event rrule until with timezone offset (#3781)
+- [feat] Add rule `no-undef` in config file validation to fix #3785 (#3786)
 
 ### Updated
 
diff --git a/js/check_config.js b/js/check_config.js
index 50cf4f44..b7912ced 100644
--- a/js/check_config.js
+++ b/js/check_config.js
@@ -54,7 +54,8 @@ function checkConfigFile () {
 				globals: {
 					...globals.node
 				}
-			}
+			},
+			rules: { "no-undef": "error" }
 		},
 		configFileName
 	);