If test problem

[D@7@K|N&]

Hey guys....

I'm having a problem with a script that I wrote. The problem that I'm having is with the 'if' test. Here's the code...

echo "Enter data."
read variable
if [-z $variable]
then
	echo "Please restart script and enter variable when asked."
	exit
else
        ...else stuff...
fi

I get an error that looks like this:

./fw-calc: [-z: not found

I've tried several different ways and all come out with basically the same result. I want the logic to read as follows...

if variable is null
then
     ...variable null code...
else
     ...variable NOT null code...
fi

Any help would be appreciated. Thanks.