Rendering Camera to PNG produces completely grey image.

The following Behaviour is attached to a new GameObject programatically, and when it's done it doesn't output a rendered image from the Camera, rather a completely grey image. What have a I done wrong or overlooked?

 using UnityEngine;
 
 namespace Cartographer
 {
 
     class MapCameraBehaviour : MonoBehaviour
     {
 
         private int size = 1000;
         private Camera camera;
         private RenderTexture renderTexture;
         private Texture2D texture;
         private bool render = false;
         private int renderFrameDelay = 0;
         private bool renderNow = false;
 
         void Log(string message)
         {
             //some logging code
         }
 
         void Awake()
         {
 
             Log("Awake()");
 
             camera = GetComponent<Camera>();
             if (camera == null)
             {
                 camera = gameObject.AddComponent<Camera>();
                 camera.nearClipPlane = 0.1f;
                 camera.farClipPlane = 2000f;
                 camera.renderingPath = RenderingPath.Forward;
                 camera.depth = 100;
                 camera.transform.position = new Vector3(0.0f, 100.0f, 0.0f);
             }
             else
             {
                 Log("Existing camera.");
             }
             if (camera != null)
             {
                 Log("Camera found.");
                 renderTexture = new RenderTexture(size, size, 0);
                 texture = new Texture2D(renderTexture.width, renderTexture.height, TextureFormat.ARGB32, false);
                 camera.targetTexture = renderTexture;
                 RenderTexture.active = renderTexture;
                 render = true;
                 renderFrameDelay = 100;
                 renderNow = false;
             }
             else
             {
                 Log("Camera not found.");
             }
 
         }
 
         void Update()
         {
             if (render)
             {
                 if (renderFrameDelay <= 0)
                 {
                     render = false;
                     renderNow = true;
                     QualitySettings.antiAliasing = 0;
                     camera.Render();
                 }
                 else
                 {
                     renderFrameDelay--;
                 }
             }
         }
 
         void OnPostRender()
         {
             if (renderNow == true)
             {
 
                 renderNow = false;
 
                 Log("OnPostRender()");
 
                 texture.ReadPixels(new Rect(0.0f, 0.0f, renderTexture.width, renderTexture.height), 0, 0);
 
                 byte[] bytes = texture.EncodeToPNG();
                 System.IO.File.WriteAllBytes(Application.dataPath + "/texture.png", bytes);
 
                 camera.targetTexture = null;
                 RenderTexture.active = null;
 
                 DestroyImmediate(texture);
                 DestroyImmediate(renderTexture);
 
                 Log("Done.");
 
             }
         }
 
     }
 
 }