How to add a tabsheet from a DLL to a pagecontrol on my current form

I have a page control on my main form and i want to add to it differents tabsheets loading them from differents DLLs.
I have created forms with a pagecontrol each of them an add it to a DLL.
Then from my main form i load the dll and get access to the form but when i try to add the tabsheet to my pagecontrol i get the error "cannot assign a tfont to tfont"