Call to a member function where() on string

im going to develop web with laravel framework. this is my code :

$valws = DB::table('layananpublik')->sum('value')->where('tahun',$tahun)->where('bulan',$stringbln)->where('kategori',$PilihKat);

and i get an error :

FatalErrorException in JobHarianController.php line 195: Call to a member function where() on string

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