user-defined hyperelastic material, field variable dependence

[VB44]

Dear all,

I have created a user-defined hyperelastic material using a UHYPER subroutine. This subroutine has 6 parameters. One of them may take two different values (val1 and val2 for example) between two steps of the FE analysis.
I wanted to use a field variable dependence as it can be done for elastic material (see the old post

http://www.eng-tips.com/viewthread.cfm?qid=105599),

but I get the following error:
***ERROR: UNKNOWN PARAMETER DEPENDENCIES
It seems that this option is not available for any kind of material. Any clue about that ?
Any idea on another way to deal with my problem ?

Thanks for your help,

VB