What is making my code that sends data to the client non-deterministic?

I have a system where a player is given an inventory when they join the game. Then, a local script that gets this inventory from the server using a remote event. However, sometimes when I test the game, the inventory is nil and nothing is transferred to the player - other times it is not nil and the inventory data is transferred. I don't really know how to fix this.

This is the function to get a player's inventory (the inventory is stored on the server)

function RetrieveInv(player)
    wait(InventoryModule:GetInventory(player):GetContents()) -- this should (intheory) wait for the inventory to exist
    Event:FireClient(player, InventoryModule:GetInventory(player):GetContents()) -- send inventory to client
end

Sometimes, the fireclient code runs, and fails because it is nil, other times it runs but an empty table is sent to the client. A lot of the time everything works though.

This is the local script that prints out the inventory

local invData

function setupInv()
    Event:FireServer()

    wait(invData)

    print(Player.Name .. "'s Items:")
    if invData ~= nil then
        for i, item in pairs(invData)do
            print("Name:" .. i .. "\nAmount: " .. item.Amount .. "\nType: " .. item.Content.ItemClass)
        end
    end

end

function updateInv(newInv)
    invData = newInv
end

Event.OnClientEvent:Connect(updateInv)

setupInv()

I know it's to do with concurrency - but I don't know how to force the game to wait for the inventory to exist before it sends anything to the client.

Adding an if statement to check if it is not nil works, but sometimes nothing is sent over, the local script stops trying to get data and the output is empty.