Trouble with RenderTexture to Texture2D

I set up a RenderTexture and rendered via ReadPixels to a Texture2D called assetThumbnail:

Code (csharp):

1. void OnGUI(){
2.         if(GUILayout.Button("Take Snapshot")) {
3.          currentCamera.Render();
4.                    
5.             float width = 0.36f;
6.             float height = 0.2f;
7.             assetThumbnail.ReadPixels(new Rect( Screen.width * width,
8.                                                   Screen.height * height,
9.                                                   Screen.width * (1.0f - width),
10.                                                   Screen.height * (1.0f - height) ),
11.                                                   0, 0, false);
12.             assetThumbnail.Apply(false);
13.       }
14.       GUILayout.Box(assetThumbnail);
15. }

The weird thing is I set the Texture2D as public variable and it shows up no problem in the inspector GUI. But it won't show up in the in-game GUI, and when I call EncodeToPNG() on the texture the image file is blank, though it still has the right size. Any help is appreciated.