setpixels throws Unity Runtime Error

I try to write an App which displays the cam-image of a smartphone flipped on a plane. Because WebCamTexture doesn't allow it to be transformed/modified I want to copy the image from "mCamera" into the Texture2D element "texture":

 Color32[] pixels = new Color32[mCamera.width*mCamera.height];
 mCamera.GetPixels32 (pixels);
 texture.SetPixels32 (pixels);

But "texture" stays empty. And when I debug and step into setpixels32 i get three errors:

• System.Exception..ctor (message="A Unity Runtime error occurred!") in /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System/Exception.cs:81

• System.SystemException..ctor (message="A Unity Runtime error occurred!") in /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System/SystemException.cs

• UnityEngine.UnassignedReferenceException..ctor () in C:\buildslave\unity\build\Runtime\Export\StackTrace.cs

Do you know what I'm doing wrong?