diff --git a/extensions/lang-english/locale/core.yml b/extensions/lang-english/locale/core.yml
index 01f584fa4..9ccc311fa 100644
--- a/extensions/lang-english/locale/core.yml
+++ b/extensions/lang-english/locale/core.yml
@@ -439,6 +439,10 @@ core:
       submit_button: Reset Password
       title: => core.ref.reset_your_password
 
+  # Translations in this namespace are used in messages output by the API.
+  api:
+    invalid_username_message: "The username may only contain letters, numbers, and dashes."
+
   # Translations in this namespace are used in emails sent by the forum.
   email:
 
diff --git a/extensions/lang-english/locale/validation.yml b/extensions/lang-english/locale/validation.yml
index 5d5df208a..d24754ec4 100644
--- a/extensions/lang-english/locale/validation.yml
+++ b/extensions/lang-english/locale/validation.yml
@@ -76,7 +76,3 @@ validation:
     name_plural: plural name
     tag_count_primary: number of primary tags
     tag_count_secondary: number of secondary tags
-
-  custom:
-    username:
-      regex: "The username may only contain letters, numbers, and dashes."