Assigning a Dynamic Variable

Dear Gurus

I want to ask about next code section. On ECC 6, the next statement is working out fine, but somehow, it's giving a short dump in 4.6c, because the second line(ASSIGN statement) is giving out SY-SUBRC = 4. and l_value is trying to access <fvalu> which is not assigned properly(or so they say in the dump).

wa_Data-ebeln = '1000000000'.

g_String = 'WA_DATA-EBELN'.

ASSIGN (g_string) TO <fvalu>.

l_value = <fvalu>.

Can anybody help to tell me what's wrong with the code? I am trying to get the value of WA_DATA-EBELN based on the field string 'WA_DATA-EBELN'. I am expecting l_value's value to be 1000000000. Is there any other way to do this kind of thing? (4.6c environment)