After turning on WordPress Debug on one of my custom themes I noticed a strict standards waring regarding the Navwalker Bootstrap menu. Error: Strict Standards: Declaration of… start_lvl() should be compatible with Walker_Nav_Menu::start_lvl(&$output, $depth = 0, $args = Array) Fix: change function start_lvl(&$output, $depth) { to function start_lvl(&$output, $depth = 0, $args = array()) {