What is "marduk-zer-x"

Marduk-zer-X (ca. 1046–1033 BC) was the 10th and penultimate king of the 2nd Dynasty of Isin, the 4th Dynasty of Babylon. The last part of his name is unknown, as the principal sources of information, the King List A and the Synchronistic King List are both damaged at this place in the sequence, hence the “x”. The reading of “zer” in his name by Poebel is almost as uncertain as the character may be MU which would correspond to šuma or similar. His Assyrian contemporary was Aššur-nasir-apli I.