Update tabel SQL statement

Hi

I try to update a database tabel, but I got an error SQL 170:

  LOOP AT g_import_tab.

    EXEC SQL.

      UPDATE MYTABLE
      flag =: g_import_tab-flag,
      lgnum =: g_import_tab-lgnum,
      tanum =: g_import_tab-tanum,
      tapos =: g_import_tab-tapos,
      squit =: g_import_tab-squit,
      lenum =: g_import_tab-lenum,
      quknz =: g_import_tab-quknz
      where lgnum = g_import_tab-lgnum
      and   tanum = g_import_tab-tanum

    ENDEXEC.

  ENDLOOP.

Can you see in the code, what I do wrong?