bash - Shell script: unexpected operator for -ne

I have a shell script where I want to check if it's being run by root. If it's not then I set the variable SUDO='sudo' to prepend to subsequent commands. That way the user gets one nice permission denied error message from the shell and if they're running with sudo, it properly does its business. However, for the following code I get:

myscript.sh: [: -ne: unexpected operator

Code is:

#!/bin/sh


SUDO=""
if [ $EUID -ne 0 ]; then
    SUDO='sudo'
fi

I am running it like: sh myscript.sh as this is not a bash script. This is not an issue of sh vs bash afaik. The shebang says /bin/sh and I'm running it with sh.