Moving Vertices, fail

I’m writing an importer for a file format that has mesh deforms (RVK’s). I’m trying to move the vertices in the mesh by a ‘morph’, an offset relative to the vertex’s current position.

the function to move the vertices is:

def addNewRVK(morph, meshobj):
    base = meshobj.data.key.blocks[0].data
    current = meshobj.data.verts
    for x in morph.vertList:
        current[x.mvid].co = base[x.mvid] + x.Coords
    meshobj.data.update()
    meshobj.insertShapeKey()
    meshobj.data.update()


base = the coordinates of the base key
current = the vertices of the mesh object
x, from morph.vertList, is a class object with the attributes mvid and Coords. mvid is the
index number of the vertice (which corresponds to verts[x].index in Blender mesh objects), and Coords is a Blender Vector Object which represents the vertex’s offset relative to the base.

Unfortunately,

current[x.mvid].co = base[x.mvid] + x.Coords

raises an exception:

AttributeError: co

It seems I can’t access the Coordinate attribute of a Blender Vertice. What am I doing wrong?