There are a number of open questions surrounding the initiation and early evolution of CMEs. Models disagree on when and how the flux rope is formed, the role and the location of magnetic reconnection, and the trigger mechanism. These questions are difficult to address observationally because the import variable -- the coronal magnetic field -- is not easily observed. We construct the coronal magnetic field of an active region prior to a major eruption using a nonlinear force-free model together with HMI vector-magnetogram data. The nonlinear force-free calculation is performed using a new implementation of the Grad-Rubin method in spherical geometry (Gilchrist and Wheatland 2014). Our method reproduces the highly twisted (non-potential) filament and sigmoid structures seen in AIA observations of the active region. We use our extrapolation to test the predictions of CME models, which predict different pre-eruption magnetic field configurations.