I've checked several times against other projects and I just cannot see what's wrong. 

I attach my code:

```
        Schema::table('datos', function (Blueprint $table) {

            $table->bigInteger('sensor_id')->change();
            $table->foreign('sensor_id')->references('id')->on('sensores')->nullable();
    });
}

/**
 * Reverse the migrations.
 *
 * @return void
 */
public function down()
{
    Schema::table('datos', function (Blueprint $table) {
        $table->dropForeign(['sensor_id']);
    });
}

```