undefined index array rand in php

i have this code and i am create a list with url images, for each image in the list i need a random image, when i launch the program, usually have this error undefined offset in line 340, how do to fixed this error

        $lista = array();
        foreach ($imagenes as $img) {
            // var_dump($img['url']);
            array_push($lista, $img['url']);
        }

        $p = array_rand($lista);

        //echo $p;
        if ($p != NULL || $p > 0) {
            echo '"Img": "' . $lista[$p]. '"' .",\n";
        } else {
            echo '"Img": "'."Sin imagen" . '"' .",\n";
        }