Index: src/wp-admin/admin.php
===================================================================
--- src/wp-admin/admin.php	(revision 34068)
+++ src/wp-admin/admin.php	(working copy)
@@ -359,7 +359,7 @@
 }
 
 $_action = wp_validate_action();
-if ( ! empty( $_action ) ) {
+if ( $_action ) {
 	/**
 	 * Fires when an 'action' request variable is sent.
 	 *
Index: src/wp-admin/network/users.php
===================================================================
--- src/wp-admin/network/users.php	(revision 34068)
+++ src/wp-admin/network/users.php	(working copy)
@@ -174,12 +174,12 @@
 
 require_once( ABSPATH . 'wp-admin/admin-header.php' );
 
-$action = wp_validate_action();
-if ( isset( $_REQUEST['updated'] ) && $_REQUEST['updated'] == 'true' && ! empty( $action ) ) {
+$_action = wp_validate_action();
+if ( isset( $_REQUEST['updated'] ) && $_REQUEST['updated'] == 'true' && $_action ) {
 	?>
 	<div id="message" class="updated notice is-dismissible"><p>
 		<?php
-		switch ( $action ) {
+		switch ( $_action ) {
 			case 'delete':
 				_e( 'User deleted.' );
 			break;
@@ -200,6 +200,7 @@
 	</p></div>
 	<?php
 }
+unset( $_action );
 	?>
 <div class="wrap">
 	<h1><?php esc_html_e( 'Users' );
Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 34068)
+++ src/wp-includes/functions.php	(working copy)
@@ -4988,18 +4988,19 @@
  *
  * @param string $action Optional. Action to validate.
  * @return string Empty string if there is no action in the request or it doesn't
- *                match the passed `$action`. Returns the [passed `$action` or
- *                request action on succcess.
+ *                match the passed `$action`. Returns the passed `$action` or
+ *                request action on success.
  */
 function wp_validate_action( $action = '' ) {
-	$r = $_REQUEST;
-	if ( ! isset( $r['action'] ) ) {
+	if ( ! is_string( $action ) || ! isset( $_REQUEST['action'] ) ) {
 		return '';
 	}
 
-	if ( ! empty( $action ) ) {
-		return $action === $r['action'] ? $action : '';
+	$raw = $_REQUEST['action'];
+
+	if ( $action ) {
+		return $action === $raw ? $action : '';
 	}
 
-	return $r['action'];
+	return is_string( $raw ) ? $raw : '';
 }
\ No newline at end of file