What MySQL returns if the search string, provided in FIELD() function, is NULL?

As we know that NULL fails equality comparison with any value hence if the search string, provided in FIELD() function, is NULL then MySQL returns 0 as output.

Example

mysql> Select FIELD(NULL,'Ram','is','good','boy');

+-------------------------------------+
| FIELD(NULL,'Ram','is','good','boy') |
+-------------------------------------+
|                                   0 |
+-------------------------------------+

1 row in set (0.00 sec)