shell script problem

I've got a shell script like this:

------------------------------
#!/bin/sh

maybe_mkdir ()
{
     if [ ! -e "${1}" ]; then
	echo "mkdir ${1}"
	mkdir "${1}"
     fi
}

maybe_mkdir tmp
------------------------------

The last call results in "test: argument expected".  Anyone see what I am 
doing wrong here?  (This is from code I would expect to work.)

David