Frame-Relay Subinterface 'cannot change link type'

I have problem with Frame Relay enable on subinterface, set 'frame-relay encapsulation' on main interface and enable subinterface with multipoint, configure use static layer 3 to layer 2 with 'frame-relay map ip'. Then a few minutes later I change point-to-point type subinterface with point-to-point, what happen ? let's check it out.

R1#
interface Serial1/0
 no ip address
 encapsulation frame-relay
 serial restart-delay 0
 no dce-terminal-timing-enable
end

R1#
interface Serial1/0.1 multipoint
 ip address 10.0.0.1 255.0.0.0
 frame-relay map ip 10.0.0.2 102 broadcast
end

R1#sh frame-relay map
Serial1/0.1 (up): ip 10.0.0.2 dlci 102(0x66,0x1860), static,
              broadcast,
              CISCO, status defined, active

R1#ping 10.0.0.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/33/60 ms


R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int s1/0.1 ?        
  multipoint      Treat as a multipoint link
  point-to-point  Treat as a point-to-point link
  

R1(config)#int s1/0.1 point-to-point 
% Warning:  cannot change link type

R1(config-subif)#do sh run int s1/0.1
Building configuration...

Current configuration : 114 bytes
!
interface Serial1/0.1 multipoint
 ip address 10.0.0.1 255.0.0.0
 frame-relay map ip 10.0.0.2 102 broadcast
end
R1(config-subif)#exit


R1(config)#default interface serial1/0.1 multipoint
Building configuration...
Interface Serial1/0.1 set to default configuration

R1(config)#do sh run int s1/0.1                    
Building configuration...
Current configuration : 40 bytes
!
interface Serial1/0.1 multipoint
end

R1(config)#int s1/0.1 point-to-point               
% Warning:  cannot change link type
R1(config-subif)#exit


R1(config)#default interface serial1/0             
Building configuration...
Interface Serial1/0 set to default configuration

R1(config)#do sh run int s1/0         
Building configuration...
Current configuration : 73 bytes
!
interface Serial1/0
 no ip address
 no dce-terminal-timing-enable
end

R1(config)#int s1/0
R1(config-if)#encapsulation frame-relay 
R1(config-if)#exit

R1(config)#int s1/0.1 point-to-point
% Warning:  cannot change link type


R1(config-subif)#do sh ip int b
Interface                  IP-Address      OK? Method Status                Protocol
Serial1/0                  unassigned      YES TFTP   up                    up      
Serial1/0.1                unassigned      YES NVRAM  up                    up      
  
R1(config-subif)#exit

R1(config)#default interface serial1/0.1 multipoint
Building configuration...
Interface Serial1/0.1 set to default configuration

R1(config)#do sh run int s1/0.1                    
Building configuration...
Current configuration : 40 bytes
!
interface Serial1/0.1 multipoint
end

I don't know why, is it bug from IOS or may be have something purpose to this condition. Now to solve this problem make default configuration subinterface, do write memory and reload the router.