Unknown variable object on map change

[Bassie]

I downloaded a map I didn't have before
I got wrong number of players while deserializing state and another error followed by a game maker error

___________________________________________
ERROR in
action number 1
of Begin Step Event
for object Client:

In script ClientBeginStep:
Error in code at line 470:
               if(player.object)
                         ^
at position 24: Unknown variable object

[MedO (smaxein)]

Did this happen just once or is/was this a regular thing? It might have been a buggy/modded server.