Texture2D.ReadPixels not working on iOS (GL error 0x0502)

I am capturing the screen using the Texture2D.ReadPixels method but when I capture the pixels, gl throws error 0x0502 indicating an invalid GL operation has occurred.

Code (C#):

 Texture2D tex = new Texture2D(Screen.width, Screen.height, TextureFormat.RGB24, false);
 Rect src = new Rect(0f, 0f, Screen.width, Screen.height);
 tex.ReadPixels(src, 0, 0, false); //Error is not thrown if this line is commented out

Actual error:

OpenGLES error 0x0502 in /[WORKING DIRECTORY]/Classes/iPhone_GlesSupport.cpp:189

Any ideas what is causing this? This is a real head scratcher for me and so far I haven't been able to turn up any similar issue on this site or through googling.