Calculate field tool in a geoprocessing service

Hi,

I'm trying to use a geoprocessing service task that contains the calculate field tool.
When I execute the model from ArcMap without using the service, everything goes right.
But when I call the gp task from the rest API, it seems that the calculate field does not work because I get a Null value in the field that should contain a numeric value.

Does calculate field work fine when used in GP service tasks?

Regards