Problem when running Servo library with thread (Protothreads)

When running two threads, one for blinking led once every second, one for sweeping the servo 180 degrees. The servo just vibrates.

When I tested adding a serial printout in the run servo for loops for the pos variable it worked but a bit jerky. Any idea what could be the problem?

#include <Arduino.h>
#include <pt.h>
#include <Servo.h>

// put function declarations here:
static struct pt pt1;
static struct pt pt2;
Servo myservo;
int pos = 0;

static int protothreadBlinkLED(struct pt *pt) {
  static unsigned long lastTimeBlink = 0;
  PT_BEGIN(pt);
  while(1) {
    lastTimeBlink = millis();
    PT_WAIT_UNTIL(pt, millis() - lastTimeBlink > 1000);
    digitalWrite(LED_BUILTIN, HIGH);
    lastTimeBlink = millis();
    PT_WAIT_UNTIL(pt, millis() - lastTimeBlink > 1000);
    digitalWrite(LED_BUILTIN, LOW);
  }
  PT_END(pt);
}

static int protoThreadRunServo(struct pt *pt) {
  static unsigned long lastServoUpdate = 0;
  PT_BEGIN(pt);
  while(1) {
    lastServoUpdate = millis();
    for(pos = 0; pos < 180; pos += 1) {
      PT_WAIT_UNTIL(pt, millis() - lastServoUpdate > 150);
      myservo.write(pos);      
    }
    for(pos = 180; pos >= 1; pos -= 1) {    
      PT_WAIT_UNTIL(pt, millis() - lastServoUpdate > 150);
      myservo.write(pos);
    }
  }
  PT_END(pt);
}

void setup() {
  // put your setup code here, to run once:
  pinMode(LED_BUILTIN, OUTPUT);
  myservo.attach(9);
  PT_INIT(&pt1);
  PT_INIT(&pt2);
}

void loop() {
  // put your main code here, to run repeatedly:
  protothreadBlinkLED(&pt1);
  protoThreadRunServo(&pt2);
}