On ordinary maps, the Player_1 should not be renamed to anything other than what it originally is. And also, there needs to be Player_2 as well. Atleast one cmoputer player is necessary to make the map valid.
However, I'm aware the Basic Scripting Guide by EA has no Player_1 or similar players. I haven't checked this in the game, however, but I feel it should work if one follows the guide carefully. I'll check it out myself, and report to you my findings.