Savedata bug

Server running version 556225 on Linux crashes with this message:

[00:00:03]: OnUpdatePurchaseStateComplete
[00:00:03]: Loading world: session/39657E2FD4589C63/0000000979
[00:00:03]: [string "local savedata = {}..."]:823: attempt to perform arithmetic on global 'nan' (a nil value)
LUA ERROR stack traceback:
        local savedata = {}
local tablefunctions = {}
tablefunctions["map_fn"] = function()
return {tiles="VlJTTgABAAAAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAA>
[00:00:03]: [string "local savedata = {}..."]:823: attempt to perform arithmetic on global 'nan' (a nil value)
LUA ERROR stack traceback:
        local savedata = {}
local tablefunctions = {}
tablefunctions["map_fn"] = function()
return {tiles="VlJTTgABAAAAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAAQABAAEAA>
[00:00:03]: Check for write access: TRUE
[00:00:03]: Check for read access: TRUE
[00:00:03]: Available disk space for save files: 133534 MB
[00:00:03]: ModIndex: Load sequence finished successfully.
[00:00:03]: Reset() returning