How to use GUI_UPLAOD?

Hello,

I am working on CRM2007 version of CRM.

I have an excel file with just one column that has phone numbers. I want to upload these phone numbers into an internal table.

I am trying to use GUI Upload in the following way

call function 'GUI_UPLOAD'

EXPORTING

filename = lv_fname

TABLES

data_tab = lt_phno

EXCEPTIONS

file_open_error = 1

file_read_error = 2.

(here lv_fname is the file path and lt_phno is the internal tabel to be populated)

But right at the call of the function I get a dump

saying 'Access via NULL object reference not possible'

Can anyone tell me how to correctly call this function 'GUI_UPLOAD'?

Regards,

Prafful.