QUESTION Raspberry Pi -bash: [: -gt: unary operator expected

[snarpatroid]

I am on a fairly old IncrediblePBX 3.11 on Raspberry Pi 'model B', because of the dirty COW exploit recently I wanted to upgrade the kernel. I've been running regular updates on the box anyway, but after I updated the kernel I now get the following when logging in:

ALWAYS RUN INCREDIBLE PBX BEHIND A SECURE, HARDWARE-BASED FIREWALL!
SECURITY MATTERS. READY TO  CHECK FOR UPDATES: /root/update-my-pi
-bash: [: -gt: unary operator expected
No mail for root

Where has the

-bash: [: -gt: unary operator expected

come from, and how can I get rid of it? I haven't noticed any change in functionality, the box is still doing everything I expect it to do, databases still work, the FreePBX GUI is still fine, the SSH login box reports all services as it did before. I'm guessing there's part of a script somewhere I need to edit just to snip a bit out?