diff --git a/wp-includes/user.php b/wp-includes/user.php
index 9d256c2612..4c43daa4ef 100644
--- a/wp-includes/user.php
+++ b/wp-includes/user.php
@@ -696,7 +696,7 @@ function get_blogs_of_user( $user_id, $all = false ) {
 	foreach ( $keys as $key ) {
 		if ( 'capabilities' !== substr( $key, -12 ) )
 			continue;
-		if ( 0 !== strpos( $key, $wpdb->base_prefix ) )
+		if ( $wpdb->base_prefix && 0 !== strpos( $key, $wpdb->base_prefix ) )
 			continue;
 		$blog_id = str_replace( array( $wpdb->base_prefix, '_capabilities' ), '', $key );
 		if ( ! is_numeric( $blog_id ) )