Post complex type to web api - json

Question

I am trying to post below complex object Inspection to a web api httppost method ,
public class Inspection
{
public string Url { get; set; }
public int HireContractLineId { get; set; }
public List<ImageInfo> Images { get; set; }
}
public class ImageInfo
{
public DateTime CapturedDateTime { get; set; }
public string ImageData { get; set; }
public string FileName { get; set; }
public string ContentType { get; set; }
}
my controller is
[ActionName("Inspection")]
[HttpPost]
public HttpResponseMessage Post([FromBody] Inspection inspection)
{
List<ImageInfo> images = inspection.Images;
for (var i = 0; i < images.Count; i++)
{
}
}
As long as I post say 5 to 6 images, the controller receives the object correctly and process it, but if I try to post with more images say 10 (all images around 200 KB), I could hit the controller , but before I reach for loop, the controller hits again and this time passed Inspection object is null and I get Null reference exception.
I guess it has something to do with the size of the passed object, but I couldn't figure out where to increase the size ,with only 5-6 Image Objects, everything works fine.
Below is how the controller is called
client.BaseAddress = new Uri(AppParameters.WebApiBaseURL);
HttpResponseMessage response =
await client.PostAsJsonAsync("api/fleetitems/Inspection", inspection);