Delete the foreign key data when the data in the foreign table is deleted

I have 2 database tables, and I want when I delete a data in the table where the foreign key is pointing (where the primary key is), the foreign key pointing to that data also be deleted.

    [HttpPost, ActionName("Delete")]
    [ValidateAntiForgeryToken]
    public async Task<ActionResult> DeleteConfirmed(int id)
    {

        Report report = await db.Reports
            .Include(i => i.RepFilters)
            .Where(i => i.ID == id).SingleAsync();

        db.Reports.Remove(report);

        await db.SaveChangesAsync();
        return RedirectToAction("Index");
    }

The foreign key is not nullable.

(the error I get)

How to do/fix that? (help me make the right title)