diff --git a/pycgtool/mapping.py b/pycgtool/mapping.py
index 506dcab..03fd2ab 100644
--- a/pycgtool/mapping.py
+++ b/pycgtool/mapping.py
@@ -417,7 +417,7 @@ def apply(self, frame: Frame, cg_frame: typing.Optional[Frame] = None):
                     coords = np.array(
                         [cg_res.atom(atom).coords for atom in bmap],
                         dtype=np.float32)
-                    bead.coords = calc_coords_weight(ref_coords, coords,
+                    bead.coords = calc_coords_weight(coords[0], coords,
                                                      bmap.weights,
                                                      unitcell_lengths)