The mod Bulldozer Equipment (1.0.0)
Error while running event BulldozerEquipment::on_player_created (ID 30)
BulldozerEquipment/control.lua:296: attempt to index upvalue 'unit_number_to_equipped_entity' (a nil value)
stack traceback:
BulldozerEquipment/control.lua:296: in function 'refresh_whether_entity_is_equipped'
BulldozerEquipment/control.lua:948: in function <BulldozerEquipment/control.lua:931>

This is the second report I've received of this issue, but I don't know how to reproduce it; the first report is at https://github.com/smcpeak/factorio-bulldozer-equipment/issues/1 .

Could you provide a save game and/or log file? See the instructions on the linked github issue for the log file. You could add them here or on github.