diff --git a/dust_extinction/shapes.py b/dust_extinction/shapes.py
index 562999f..e15ce36 100644
--- a/dust_extinction/shapes.py
+++ b/dust_extinction/shapes.py
@@ -121,8 +121,9 @@ def _curve_F99_method(
 
         spline_x = np.concatenate([x_splineval_optir, x_splineval_uv])
         spline_y = np.concatenate([y_splineval_optir, y_splineval_uv])
-        spline_rep = interpolate.splrep(spline_x, spline_y)
-        axav[indxs_opir] = interpolate.splev(x[indxs_opir], spline_rep, der=0)
+        axav[indxs_opir] = interpolate.CubicSpline(spline_x, spline_y, bc_type='natural')(x[indxs_opir])
+        #spline_rep = interpolate.splrep(spline_x, spline_y)
+        #axav[indxs_opir] = interpolate.splev(x[indxs_opir], spline_rep, der=0)
 
     # return A(x)/A(V)
     return axav
diff --git a/dust_extinction/tests/test_corvals.py b/dust_extinction/tests/test_corvals.py
index 0c29e4b..e11e0e4 100644
--- a/dust_extinction/tests/test_corvals.py
+++ b/dust_extinction/tests/test_corvals.py
@@ -57,7 +57,7 @@
         "Rv": 3.1,
         "x_values": [0.377, 0.820, 1.667, 1.828, 2.141, 2.433, 3.704, 3.846],
         "y_values": np.array([0.185, 0.772, 2.688, 3.055, 3.805, 4.315, 6.456, 6.781]) / 3.1,
-        "atol": 1e-3,
+        "atol": 4e-3,
         # from Fitzpatrick (1999) Table 3
         # keep optical from Fitzpatrick (1999),
         # replce NIR with Fitzpatrick (2004) function for Rv=3.1: