Laravel Schema onDelete set default

I have two tables with a one-to-one relationship, set by a foreign key constraint. I want to set the onDelete rule to 'set default', meaning that when a row on the foreign table is dropped, the reference value reverts to its default value. Here is my code:

Tours Table:

    Schema::create('tours', function (Blueprint $table) {
        $table->increments('id')->unsigned()->index();
        $table->integer('grade')->unsigned()->default(1);
        $table->timestamps();
    });

Grades Table:

    Schema::create('grades', function(Blueprint $table){
        $table->increments('id')->unsigned()->index();
        $table->string('name');
        $table->timestamps();
    });

Set Foreign Key:

    Schema::table('tours', function (Blueprint $table) {
        $table->foreign('grade')->references('id')->on('grades')->onDelete('set default');
    });

When I run my migrations, it works- no errors. However, looking at the table in HeidiSQL, it hasn't worked properly; the foreign key has been set, but the onDelete property just shows 'NO ACTION'.

Subsequently when I delete a row in the grades table, I get a foreign key constraint error.

What am I doing wrong?

via Chebli Mohamed