Duck egg question

I recently bought a male/female muscovy and a male/female peking duck. Only the pekings are laying eggs as of now. My question is that when i crack the egg the shell breaks faily well but the lining inside the shell is very tought and will not break without plunging my thumb inside the shell. It has been years since i've had expreience with duck eggs and i need to know if this is normal or if this is indicative of something missing or too much of something in their diet? The eggs are otherwise great to eat and i see nothing that is amiss.