diff --git a/docs/en/04_Changelogs/4.0.0.md b/docs/en/04_Changelogs/4.0.0.md
index 3b5caeba6..5dddca074 100644
--- a/docs/en/04_Changelogs/4.0.0.md
+++ b/docs/en/04_Changelogs/4.0.0.md
@@ -418,6 +418,13 @@ variable set-up process. This generally encouraged bad practice and should be av
 logic early in the bootstrap, this is best placed in the `_config.php` files.
 
 Note also that `$_FILE_TO_URL_MAPPING` has been removed and replaced with `SS_BASE_URL` env var.
+This url must be an absolute url with an optional protocol. The following are valid, for example:
+
+```
+SS_BASE_URL="http://localhost/"
+SS_BASE_URL="https://localhost/"
+SS_BASE_URL="//localhost/"
+```
 
 The global values `$database` and `$databaseConfig` have been deprecated, as has `ConfigureFromEnv.php`
 which is no longer necessary.
diff --git a/src/Control/ContentNegotiator.php b/src/Control/ContentNegotiator.php
index 8e5f621c3..fd99dc69c 100644
--- a/src/Control/ContentNegotiator.php
+++ b/src/Control/ContentNegotiator.php
@@ -136,7 +136,7 @@ class ContentNegotiator
         }
 
         $negotiator = new ContentNegotiator();
-        $negotiator->$chosenFormat( $response );
+        $negotiator->$chosenFormat($response);
     }
 
     /**
diff --git a/src/includes/constants.php b/src/includes/constants.php
index a6419097c..27fada010 100644
--- a/src/includes/constants.php
+++ b/src/includes/constants.php
@@ -72,6 +72,20 @@ if (!getenv('SS_IGNORE_DOT_ENV')) {
     });
 }
 
+// Validate SS_BASE_URL is absolute
+if (getenv('SS_BASE_URL') && !preg_match('#^(\w+:)?//.*#', getenv('SS_BASE_URL'))) {
+    call_user_func(function () {
+        $base = getenv('SS_BASE_URL');
+        user_error(
+            "SS_BASE_URL should be an absolute url with protocol "
+            . "(http://$base) or without protocol (//$base)",
+            E_USER_WARNING
+        );
+        // Treat as protocol-less absolute url
+        $base = '//' . $base;
+        putenv("SS_BASE_URL=$base");
+    });
+}
 
 if (!defined('BASE_URL')) {
     define('BASE_URL', call_user_func(function () {