F4 in ALV

Hi,

I have created a ALV grid in ABAP OOP.

Want to give F4 help fo rthe material which is key coulmn in the grid. What all the values i need to set in the fieldcatalog for this.

F4AVAILABL = X

checktable = "MARA'.

ref_table = 'MARA'.

ref_field = 'MATNR'

VALEXI = X

When ever i enter a material in the grid, i get the meesage as invalid value.

Is there any problem in the fieldcatalog as mentioned above

Pls help

Tahnsk