RenderTextures are black on Galaxy Nexus

I'm trying to save a RenderTexture as a Texture2D for saving/reuse, using this technique from the documentation:

 RenderTexture currentRT = RenderTexture.active;
 RenderTexture.active = cam.targetTexture;
 cam.Render();
 Texture2D image = new Texture2D(cam.targetTexture.width, cam.targetTexture.height);
 image.ReadPixels(new Rect(0, 0, cam.targetTexture.width, cam.targetTexture.height), 0, 0);
 RenderTexture.active = currentRT;
 return image;

In the editor and on a Nexus 7 tablet (Android 4.2.2, the code works fine, but on a Samsung Galaxy Nexus phone (Android 4.3) the textures are always black. Is there some kind of texture format setting I'm missing that would result in the textures not appearing?