diff --git a/src/wp-includes/ms-functions.php b/src/wp-includes/ms-functions.php
index bc95f8b0dd..ab561af548 100644
--- a/src/wp-includes/ms-functions.php
+++ b/src/wp-includes/ms-functions.php
@@ -494,7 +494,7 @@ function wpmu_validate_user_signup($user_name, $user_email) {
 	if ( strlen( $user_name ) < 4 )
 		$errors->add('user_name',  __( 'Username must be at least 4 characters.' ) );
 
-	if ( strpos( ' ' . $user_name, '_' ) != false )
+	if ( strpos( ' ' . $user_name, '_' ) !== false )
 		$errors->add( 'user_name', __( 'Sorry, usernames may not contain the character &#8220;_&#8221;!' ) );
 
 	// all numeric?