if command "cat /dev/net/tun" result $string then

I'm creating a script which check if a VPS do have TUN driver enabled. The check command is :

cat /dev/net/tun

if it return:

cat: /dev/net/tun: File descriptor in bad state

the module is enabled. otherwise return ERROR.

Here is my script:

tunstring="File descriptor in bad state"
if cat /dev/net/tun | grep -q "$tunstring"; then
    echo "GOOOOOD"
else
    echo "ERROR"
fi

I get ERROR message.

I tried the same script with a text file and it worked...