During the Last Interglacial period, spanning 129 000 to 116 000 years ago, the Arctic was considered warmer than during the preindustrial period. Many climate models do not simulate an ice-free Arctic Ocean in summer, as suggested by recent reconstructions. Here, we examine the importance of how the liquid or solid phase of cloud particles is determined in models. It is found that the representation of cloud phase indeed has a substantial impact on the simulation of summer sea ice cover.