Warning in SLIN:'Does not test for equality'

Hi i get the following error

In "SELECT SINGLE ...", the WHERE condition for the key field "MATNR" does not

test for equality. Therefore, the single record in question may not be unique.

for the following code

SELECT SINGLE * FROM zgd_phant_status

INTO t_phant_status

WHERE sellable = p_l_matnr

AND opsys = g_opsys

AND rellvl = g_rel

AND serlvl = g_ser

AND lang = g_lang.