Spline2D with 1D Array z values

Seems not to work, here my simple example that does not work.

using Dierckx
x=[0.7, 5.3, 3.3, 7.8, 2.2, 7.3]
y=[0.9, 0.6, 3.1, 2.1, 4.8, 4.2]
z=[3., 4., 7., 12., 18., 9.]
Spline2D(x, y, z)

error message is:
LoadError: Error on entry, no approximation returned. The following conditions
must hold:
xb<=x[i]<=xe, yb<=y[i]<=ye, w[i]>0, i=0..m-1
If iopt==-1, then
xb