[Jprogramming] Fold -- can't get it to work

[Piet de Jong]

Started looking at fold — seems a very useful new verb.
However couldn’t get it to work  

Engine: j901/j64avx2/darwin
Release-e: commercial/2020-01-29T12:41:32
Library: 9.01.21
Qt IDE: 1.8.6/5.12.7(5.12.7)
Platform: Darwin 64
Installer: J901 install
InstallPath: /applications/j901

Here is the what happens when I run the example from the vocabulary

   u
3 : 0
z=. -y
z [smoutput ' u' ; y ; '-->' ; z
)
v
4 : 0
z=. y + 0.01
z [smoutput x ; 'v' ; y ; '-->' ; z
)
y
100 0.1 1.1 2.1 3.1
u F:.v y
|domain error
| u F:.v y

Any help much appreciated.