It appears to be a very old bug that only affects the player when the starting slave is not assigned as a sibling(though that has its own bug associated with it). Unlike slaves, the player has no handling of not having any relations data. The bug will go away when the player character is directly related to a pregnancy, as the unborn child will generate the relations data.
The only interesting part is that prior to 1.0d it would generate an error message but not crash. This is because the game now defaults to relying on logs(stored in AppData) rather than debug mode.