Runtime IndexOutOfRangeException when testing on iPhone

I have a script that fetches JSON data from a REST api, and translates it using "JsonUtility.FromJson". The scripts is the following:

 UnityWebRequest www = UnityWebRequest.Get("http://my_url/getNearPlanes?longitude="+myLocation.y+"&latitude="+myLocation.x+"&altitude="+myLocation.z+"&maxDistance=300");
         yield return www.Send();
 
         if(www.isError) {
             Debug.Log(www.error);
         }
         else {
             MyPlanes planeList = JsonUtility.FromJson<MyPlanes>(www.downloadHandler.text);
 
             for (int i = 0; i < planeList.planes.Count; i++) {
                 MyPlane mp = planeList.planes [i];
 
                 float tempLat = mp.latitude - myLocation.x;
                 float tempLong = mp.longitude - myLocation.y;
                 float tempAlt = mp.altitude - myLocation.z;
 
                 Instantiate(airPlane, new Vector3 (tempLat,tempAlt,tempLong), Quaternion.identity, this.transform);
             }
         }

I have a MyPlanes object:

 [Serializable]
 public class MyPlanes{
     public List<MyPlane> planes;
 }

and a MyPlane struct:

 [Serializable]
 public struct MyPlane{
     public string icao24;
     public string callsign;
     public string origin_country;
     public float time_position;
     public float time_velocity;
     public float longitude;
     public float latitude;
     public float altitude;
     public bool on_ground;
     public float velocity;
     public float heading;
     public float vertical_rate;
     public int[] sensors;
 }

When running this code within Unity it all works fine. The problem is that, when I build for iOS Xcode gives me the following error:

 IndexOutOfRangeException: Array index is out of range.
   at System.Collections.Generic.List`1[T].get_Item (Int32 index) [0x00000] in <filename unknown>:0 
   at AirPlanesController+<Start>c__Iterator0.MoveNext () [0x00000] in <filename unknown>:0 
   at UnityEngine.SetupCoroutine.InvokeMoveNext (IEnumerator enumerator, IntPtr returnValueAddress) [0x00000] in <filename unknown>:0 
  
 (Filename: currently not available on il2cpp Line: -1)

When I write print(planeList.planes.Count) I get the correct number of elements (list is not empty). The error seems to occur when I want to fetch a specific element in the list: planeList.planes[i].

I guess the issue relates to something around the struct type. Any suggestions?

Thanks in advance.