Remove "No LSB modules are available." message from 'lsb_release -a'

I try to write a script to get the version of my distro so that I can pass it to a variable. The following command is what I wrote to achieve the result.

lsb_release -ar | grep -i release | cut -s -f2

The unwanted output:

No LSB modules are available.
18.04

As you can see, the No LSB modules are available message is the unwanted part. Since I prefer my script to be portable across servers, I don't want to install any extra packages beside utilizing the lsb_release -a command.