In singleplayer it spits the error 'event': real number expected got nil to the console, but doesn't hurt anything. On my multiplayer server however, the error occurs when I first connect, and the server quits and restarts.

61.238 Info ServerMultiplayerManager.cpp:997: UpdateTick(55118593) Serving map(/opt/factorio/server/temp/mp-save-0.zip) for peer(1) size(63726891) crc(4131983663)
  61.238 Info ServerMultiplayerManager.cpp:780: updateTick(55118593) changing state from(InGameSavingMap) to(InGame)
  61.322 Info ServerMultiplayerManager.cpp:930: updateTick(55118593) received stateChanged peerID(1) oldState(ConnectedWaitingForMap) newState(ConnectedDownloadingMap)
  64.306 Info ServerMultiplayerManager.cpp:930: updateTick(55118593) received stateChanged peerID(1) oldState(ConnectedDownloadingMap) newState(ConnectedLoadingMap)
  64.353 Script @__stdlib__/stdlib/core.lua:50: 'event': real number expected got nil.
  64.353 Error MainLoop.cpp:1281: Exception at tick 55118593: The mod Picker Inventory Tools (1.1.12) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event PickerInventoryTools::on_tick (ID 0)
__stdlib__/stdlib/event/event.lua:368: 'event': real number expected got nil.
stack traceback:
        [C]: in function 'error'
        __stdlib__/stdlib/event/event.lua:368: in function 'dispatch_event'
        __stdlib__/stdlib/event/event.lua:438: in function <__stdlib__/stdlib/event/event.lua:396>
  64.353 Error ServerMultiplayerManager.cpp:91: MultiplayerManager failed: "The mod Picker Inventory Tools (1.1.12) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event PickerInventoryTools::on_tick (ID 0)
__stdlib__/stdlib/event/event.lua:368: 'event': real number expected got nil.
stack traceback:
        [C]: in function 'error'
        __stdlib__/stdlib/event/event.lua:368: in function 'dispatch_event'
        __stdlib__/stdlib/event/event.lua:438: in function <__stdlib__/stdlib/event/event.lua:396>"
  64.353 Info ServerMultiplayerManager.cpp:780: updateTick(55118593) changing state from(InGame) to(Failed)
  64.353 Quitting: multiplayer error.

It seems to be a transient sort of error. In singleplayer it threw the error about 20 times at 1/second, and then stopped. Then I could save and load that save on the server and it would work.