diff --git a/core/modules/server/authenticators/header.js b/core/modules/server/authenticators/header.js
index 6d21cf023..4d3894f1c 100644
--- a/core/modules/server/authenticators/header.js
+++ b/core/modules/server/authenticators/header.js
@@ -14,7 +14,7 @@ Authenticator for trusted header authentication
 
 function HeaderAuthenticator(server) {
 	this.server = server;
-	this.header = server.get("authenticated-user-header");
+	this.header = server.get("authenticated-user-header").toLowerCase();
 }
 
 /*