Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, bool givennilai.php on line 67

Berikut beberapa kode nya

                          <?php
				if($filter){
				 $sql = mysqli_query($koneksi, "SELECT * FROM
                                 tbl_mahasiswa WHERE grade='$filter' ORDER BY nim ASC");
					}else{
			         $sql = mysqli_query($koneksi, "SELECT WHERE
                                 tbl_mahasiswa.nim='nim_nilai'
                                 FROM tbl_mahasiswa
                                 INNER JOIN nilai_mahasiswa
                                 ON tbl_mahasiswa.nim_nilai = nilai_mahasiswa.nim
                                 WHERE tbl_mahasiswa.nim");
					}
				LINE 67 if(mysqli_num_rows($sql) == 0){