UCS saving process failed. Can't fork

It appears the issue is with trying to use a passphrase to encrypt the UCS file.

With no private keys and not encrypted it works

[/S1-green-P:Active:In Sync] ~  tmsh save sys ucs /var/tmp/DJ/$HOSTNAME"."$(date +%Y%m%d).ucs no-private-key
Saving active configuration...
/var/tmp/DJ/servername.20190402.ucs is saved.
Please note that UCS files saved with the no-private-key option may load with validation errors.

With private keys and encrypted it fails

[/S1-green-P:Active:In Sync] ~  tmsh save sys ucs /var/tmp/DJ/$HOSTNAME"."$(date +%Y%m%d).ucs passphrase g00b3r
Saving active configuration...
Can't fork at /usr/local/bin/im line 305.
/var/tmp/configsync.spec: Error creating package

WARNING:There are error(s) during saving.
            Not everything was saved.
            Be very careful when using this saved file!

Error creating package
Error during config save.
Unexpected Error: UCS saving process failed.

Version running is:

[/S1-green-P:Active:In Sync] ~  tmsh show sys version

Sys::Version
Main Package
  Product     BIG-IP
  Version     12.1.3.5
  Build       0.0.10
  Edition     Point Release 5
  Date        Mon May 21 17:36:20 PDT 2018