ReadPixels returns empty texture

I try to make a screenshot and put it into a texture to use it in my app(I do not need to save ot somewhere on disc). I am trying to use ReadPixels without RenderTexture, because with RenderTexture it returns a low-quality image, and I need a perfect representation of what's on the screen. I have two cameras in my scene, and two canvases attached to these cameras(each canvas is attached to different cameras), and cameras have different culling masks, so one camera renders background, another renders other dinamically created UI image objects. So, here's my code:

 IEnumerator OnPostRender()
 {
     yield return new WaitForEndOfFrame();
     Texture2D snapshot = new Texture2D(Screen.width, Screen.height, TextureFormat.ARGB32, false);
     snapshot.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0);
     snapshot.Apply();
     myImage.GetComponent<Image>().sprite = Sprite.Create(snapshot, new Rect(0, 0, snapshot.width, snapshot.height), Vector2.zero);
 }

I am calling this function in LateUpdate with a line StartCoroutine(OnPostRender());, but the function gives me an empty texture with current screen resolution. Even if I change my scene so that both canvases are rendered in the main camera and the second camera is off, the texture is still empty. What am I doing wrong?