diff --git a/gwcs/wcs/_wcs.py b/gwcs/wcs/_wcs.py
index 57221488..11dc24af 100644
--- a/gwcs/wcs/_wcs.py
+++ b/gwcs/wcs/_wcs.py
@@ -2763,14 +2763,14 @@ def _calc_approx_inv(self, max_inv_pix_error=5, inv_degree=None, npoints=16):
             if isinstance(transform, projections.Projection):
                 sky2pix_proj = transform
                 break
-        if sky2pix_proj.__name__.startswith("Pix2Sky"):
+        if sky2pix_proj.__class__.__name__.startswith("pix2sky"):
             sky2pix_proj = sky2pix_proj.inverse
         lon_pole = _compute_lon_pole((crval1, crval2), sky2pix_proj)
         ntransform = (
             (Shift(crpix[0]) & Shift(crpix[1]))
             | self.forward_transform
             | RotateCelestial2Native(crval1, crval2, lon_pole)
-            | sky2pix_proj()
+            | sky2pix_proj
         )
 
         # standard sampling: