restrict ssh access to /bin and using bash as a shell

I want to restrict my admin user to /bin directory while permitting to use bash as a shell via ssh. this is in my sshd_config file:

Match User admin    
   ChrootDirectory /bin
   X11Forwarding no
   ForceCommand scponly
   #PermitTunnel no
   AllowTcpForwarding no

in /etc/passwd:

admin:x:1001:100:,,,:/:/bash

when i use this command: ssh admin@192.168.1.20

this error appears: /bash: No such file or directory Connection to 192.168.200.242 closed.

if i change scponly to internal-sftp in sshd_config, all binary files exist (for example, bash) in / directory via sftp. why i can't use /bash as a shell after changing to scponly?

Can you help me?

Regards, siavash