diff --git a/plugin/core/src/main/java/org/elasticsearch/xpack/ml/action/PostCalendarEventsAction.java b/plugin/core/src/main/java/org/elasticsearch/xpack/ml/action/PostCalendarEventsAction.java
index 2c97516b3f5..eabec4a82e2 100644
--- a/plugin/core/src/main/java/org/elasticsearch/xpack/ml/action/PostCalendarEventsAction.java
+++ b/plugin/core/src/main/java/org/elasticsearch/xpack/ml/action/PostCalendarEventsAction.java
@@ -84,7 +84,7 @@ public class PostCalendarEventsAction extends Action<PostCalendarEventsAction.Re
             this.scheduledEvents = ExceptionsHelper.requireNonNull(scheduledEvents, EVENTS.getPreferredName());
 
             if (scheduledEvents.isEmpty()) {
-                throw ExceptionsHelper.badRequestException("at least 1 event is required");
+                throw ExceptionsHelper.badRequestException("At least 1 event is required");
             }
         }
 
diff --git a/plugin/src/test/resources/rest-api-spec/test/ml/calendar_crud.yml b/plugin/src/test/resources/rest-api-spec/test/ml/calendar_crud.yml
index b604e779575..37c1d9ce21a 100644
--- a/plugin/src/test/resources/rest-api-spec/test/ml/calendar_crud.yml
+++ b/plugin/src/test/resources/rest-api-spec/test/ml/calendar_crud.yml
@@ -596,7 +596,7 @@
 "Test post calendar events given empty events":
 
   - do:
-      catch: /at least 1 event is required/
+      catch: /At least 1 event is required/
       xpack.ml.post_calendar_events:
         calendar_id: "foo"
         body: >