Field symbol problem

Hi,

I am getting this short dump error .please advise what i need to do.

Error analysis

A new value is to be assigned to the field "<GFS_F1>", although this field is

entirely or partly protected against changes.

The following are protected against changes:

- Character literals or numeric literals

- Constants (CONSTANTS)

- Parameters of the category IMPORTING REFERENCE for functions and

methods

- Untyped field symbols not yet assigned a field using ASSIGN

- TABLES parameters if the actual parameter is protected against changes

- USING reference parameters and CHANGING parameters for FORMs, if the

actual parameter is protected against changes and

- Accesses using field symbols if the field assigned using ASSIGN is

protected (or partially protected, e.g. key components of an internal

table with the type SORTED or HASHED TABLE) against changes

- Accesses using references, if the field bound to the reference is

protected (or partially protected) against changes

- External write accesses to READ-ONLY attributes,

- Content of a shared object area instance accessed using a shared lock

(ATTACH_FOR_READ).

The problem is with this line <gfs_f1> = i_nodata. of below code.

FORM init_structures USING tabname tab i_nodata.
  REFRESH git_nametab.
  CALL FUNCTION 'NAMETAB_GET'
    EXPORTING
      langu          = sy-langu
      tabname        = tabname
    TABLES
      nametab        = git_nametab
    EXCEPTIONS
      no_texts_found = 1.
  LOOP AT git_nametab INTO gwa_nametab.
    CLEAR gfd_char.
    gfd_char(2)    = 'I_'.
    gfd_char+2(5)  = gwa_nametab-tabname.
    gfd_char+7(1)  = '-'.
    gfd_char+8(10) = gwa_nametab-fieldname.
    ASSIGN (gfd_char) TO <gfs_f1>.
    <gfs_f1> = i_nodata.
  ENDLOOP.
ENDFORM.                    "init_structures

Regards