Let's encrypt problem

I’ve tried to request a certificate from let’s encrypt using the tab on the manage ssl certifcate page for a domain but it comes up with the following error:

Failed to request certificate :
  File "/usr/libexec/webmin/webmin/acme_tiny.py", line 31
    pub_exp = "0{0}".format(pub_exp) if len(pub_exp) % 2 else pub_exp
SyntaxError: invalid syntax

Any ideas what I may be doing wrong?