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.

You can't use any YieldFunctions in C callbacks, particularly if they're expected to return. Both InvokeServer and GetAsync are YieldFunctions, meaning they can not be used.

Your solution in this case is possibly to get it asynchronously. Using a RemoteEvent, send a request to the server, and then wait for the response. This should solve the C boundary error because it's no longer a C callback, as it is handled by an event connection.