AutoCAD Inventor :: Create Workplane From Selected Face?

I am working on a project were I need to select a face and create a workplane out of such face, see the figure attach. my language of choice is C#. 

Here is what I think it should be done, my proble is that the oWork (the workplane I just created remains null) and the whole project crashes.
 
private Inventor.Application mApp = null;.....if (((mApp.ActiveDocument != null))){
if ((mApp.ActiveDocument.DocumentType == DocumentTypeEnum.kPartDocumentObject)){
PartComponentDefinition oPartCom = mApp.ActiveDocument as PartComponentDefinition;
PartDocument oDoc = mApp.ActiveDocument as PartDocument;if ((oDoc.SelectSet[1]) is Face){
System.Windows.Forms.MessageBox.Show("You just selected a surface", "Surface Evaluator");Face oFace = oDoc.SelectSet[1] as Face;
WorkPlane oWorkpl = oFace as WorkPlane;
PlanarSketch oSketch = oDoc.ComponentDefinition.Sketches.Add(oWorkpl, false);
//WHAT AM I DOING WRONG?
}}}