diff --git a/wp-includes/functions.php b/wp-includes/functions.php
index b7b010941..57488f2b3 100644
--- a/wp-includes/functions.php
+++ b/wp-includes/functions.php
@@ -1824,13 +1824,19 @@ function is_blog_installed() {
 	$tables = $wpdb->get_col('SHOW TABLES');
 	$wpdb->suppress_errors( $suppress );
 
-	$wp_tables = $wpdb->tables( 'all' );
+	$wp_tables = $wpdb->tables();
 	// Loop over the WP tables.  If none exist, then scratch install is allowed.
 	// If one or more exist, suggest table repair since we got here because the options
 	// table could not be accessed.
 	foreach ( $wp_tables as $table ) {
 		// If one of the WP tables exist, then we are in an insane state.
 		if ( in_array( $table, $tables ) ) {
+			// The existence of custom user tables shouldn't suggest an insane state or prevent a clean install.
+			if ( defined( 'CUSTOM_USER_TABLE' ) && CUSTOM_USER_TABLE == $table )
+				continue;
+			if ( defined( 'CUSTOM_USER_META_TABLE' ) && CUSTOM_USER_META_TABLE == $table )
+				continue;				
+
 			// If visiting repair.php, return true and let it take over.
 			if ( defined('WP_REPAIRING') )
 				return true;