Unknown RemoteFunction error?

Error: attempt to yield across metamethod/C-call boundary

Code on client:

local RequestSuccess, Response = pcall(function()
    return MiddleMan:InvokeServer('CheckPurchased',itemKey)
end)

Code on server (simplified):

function MiddleMan.OnServerInvoke (client,mode,...)
    local args = {...}
    if mode=='CheckPurshased' then
        local purchased = PurchasesStorage:GetAsync(tostring(client.userId) .. '_' .. tostring(args[1]))
        return (purchased==1)
    end
end

NOTES:

• MiddleMan is a RemoteFunction in ReplicatedStorage;

• itemKey is a string;

• PurchasesStorage is a GlobalDataStore;

• Data saved on PurchasesStorage is always saved as:

  • Key: userId_itemKey;
  • Value: 1

Why is this happening? How to fix it?

Any help is highly appreciated.