[Therion] Survex Terrain model problem

Hi I was experimenting with the .3d model exported from Therion to Survex. The Survex viewer (Aven) has an option to display the terrain too. When I go to View > Terrain I get an error "No coordinate system specified in survey data". In the Therion data there is a coordinate system specified, fixed station as well as terrain data and it is displayed in Lox. Has anyone seen this error or had success in showing the terrain in Survex? Vladimir