xenserver-pv-tools post-install script errors

The unmount_halted_xstools.sh script that is run at post-installation of the xenserver-pv-tools package (either initial installation or RPM update) can display errors:

/opt/xensource/libexec/unmount_halted_xstools.sh: line 10: [: ==: unary operator expected 

Here's the line in question:

            if [ $(xe vm-list uuid=${VMID} params=power-state --minimal) == 'halted' ]; then 

The error message suggests that

(xe vm-list uuid=${VMID} params=power-state --minimal) 

may evaluate to nothing, which is a case that should be taken into account in the script.