diff --git src/wp-includes/class-wp-query.php src/wp-includes/class-wp-query.php
index 21127dc755..734ab78a21 100644
--- src/wp-includes/class-wp-query.php
+++ src/wp-includes/class-wp-query.php
@@ -2431,6 +2431,8 @@ class WP_Query {
 			$q['author']      = get_user_by( 'slug', $q['author_name'] );
 			if ( $q['author'] ) {
 				$q['author'] = $q['author']->ID;
+			} else {
+				return array();
 			}
 			$whichauthor .= " AND ({$wpdb->posts}.post_author = " . absint( $q['author'] ) . ')';
 		}