In a service provider network, area 0 connects to a non-backbone area 10 through an ABR named R2. An ASBR named R5 resides inside area 10 and redistributes external routes into OSPF. A router in area 0 (R1) can see the Type 5 external LSAs in its database but shows the external routes as unreachable. Which condition explains why R1 cannot install these external routes?