The Dark Ages

Just watched the Dark Ages from the History Channel. It was very well done and held my interest.However,I detected a serious historical error.Specifically, it stated with colored map that Poland was invaded and made part of Charlamagne's Empire.While some western Slavs on the west side of the Oder were subdued and became allies of Charlamagne ( Sorbs and Wilzians) ,which is now Germany,Charlamagne never conquered the land east of the Oder river which is now present day Poland. In the interests of accuracy I point out this error.Charlamagne Christianized mostly all of Western and Central Europe up to the Oder river by 813 but Poland remained pagan until the 10th century. Dr Marc