diff --git a/setup.sh b/setup.sh
index 6b81b6e..68cdd04 100755
--- a/setup.sh
+++ b/setup.sh
@@ -46,7 +46,11 @@ case "$primary_region" in
         ;;
 esac
 
-prompt admin_username 'Forgejo admin username' 'admin'
+prompt admin_username 'Forgejo admin username'
+if [ "${admin_username,,}" = admin ]; then
+    printf 'The username "admin" is reserved by Forgejo. Choose another username.\n' >&2
+    exit 1
+fi
 case "$admin_username" in
     -*|*-|*[!A-Za-z0-9._-]*)
         printf 'The Forgejo username may only contain letters, numbers, ., _, and internal hyphens.\n' >&2