title: wifi_fpm_do_sleep(0xFFFFFFF) returns -1;
body: <p>I cannot do light sleep on my NodeMCU v3 ESP8266, because the function <code>wifi_fpm_do_sleep()</code> returns -1. Can you help me to find why there is an error?</p>
  <pre><code>void light_sleep() {
    wifi_fpm_set_sleep_type(LIGHT_SLEEP_T);
    gpio_pin_wakeup_enable(GPIO_ID_PIN(gpio_read), GPIO_PIN_INTR_HILEVEL);
    wifi_fpm_open();
    WiFi.disconnect();
    WiFi.mode(WIFI_OFF);
    Serial.print(&quot;WiFi Sleep\t&quot;);
    WiFi.forceSleepBegin();
    int err = wifi_fpm_do_sleep(0xFFFFFFF);
    Serial.print(&quot;ERROR: &quot;);
    Serial.println(err);
  }
  </code></pre>
  <p>Thank you.</p>