[QGIS-Developer] Processing data defined parameters in modeler

[matteo]

Hi devs,

I'm playing with the Processing modeler and I noticed something weird
when using data defined options.

The simple algorithm "Buffer" now accepts the data defined parameter for
the distance. So I have a simple model with vector, vector field and
buffer algorithm.

In order to define the distance of the buffer from the field I have to
use the expression dialog and put the @vector-field-name parameter
(Field and Value menu shows only one NULL entry)

But it seems  it is not taken for the calculation of the distance.

Outside the modeler it works perfect.

Am I using this option in the wrong way?

Thanks

Matteo