Backup-DhcpServer & -ErrorVariable | Variable is null after an error

I’m trying to understand why I’m not getting an error returned in the specified ErrorVariable with Backup-DhcpServer. Below is the code I’ve written.

The same happens when I issue the commands manually. The variable I specify to be used remains empty. This seems to be unique to Backup-DhcpServer.

In the logfile I have the script outputting errors to is an example of where the output should be. There should be three lines. The ErrorVariable output should be between these two lines. However, the second line is for the next server in the iteration and is not related to the first.

ERROR: Failed to backup SERVER1. Verify that it should be an authorized DHCP server in AD or if the authorized DHCP server entry should be removed from AD.
ERROR: SERVER2 is offline. Verify that it should be an authorized DHCP server in AD or if the authorized DHCP server entry should be removed from AD.

What am I missing?

Please do not post images of code as this is not helpful at all. Instead post the the plain text and format it as code using the preformatted text button.

Thanks in advance.