@@ -27,7 +27,7 @@ def _default_loss_per_interval(interval, scale, function_values):

         loss = dx

     else:

         dy = (y_right - y_left) / y_scale

-        if hasattr(dy, '__len__') and len(y) > 1:

+        if hasattr(dy, '__len__') and len(dy) > 1:

             loss = np.hypot(dx, dy).max()

         else:

             loss = math.hypot(dx, dy)