Cannot get RenderTexture to clear garbage.

I am trying to take a transparent screenshot. I have it like half the way there. The problem is that sometimes my RenderTexture has garbage in it. DiscardContents seems to do nothing. Right now I am trying to use GL.CLear which seems to work 50% of the time. Here is my method. How do I ensure the RenderTexture contents are always initially cleared?

     private IEnumerator SaveScreenshotToFile(string fileName)
     {
         yield return null;

         int resWidth = 1024;
         int resHeight = 1024;

         Camera camera = Camera.main;
         RenderTexture rt = new RenderTexture(resWidth, resHeight, 32);

         RenderTexture.active = rt;
         yield return new WaitForEndOfFrame();
         GL.Clear(false, true, Color.clear);
         yield return new WaitForEndOfFrame();
         RenderTexture.active = null;

         camera.clearFlags = CameraClearFlags.Depth;
         camera.targetTexture = rt;
         yield return new WaitForEndOfFrame();

         RenderTexture.active = rt;

         Texture2D texture = new Texture2D(resWidth, resHeight, TextureFormat.ARGB32, false);
         texture.ReadPixels(new Rect(0, 0, resWidth, resHeight), 0, 0);

         camera.targetTexture = null;
         RenderTexture.active = null; 

         byte[] bytes = texture.EncodeToPNG();
         System.IO.File.Delete(fileName);
         System.IO.File.WriteAllBytes(fileName, bytes);
         return texture;
     }