Why do I get the error "Unary Operator Expected" when constructing this simple if else statement?

I'm making a program, lab5, to test if the argument is a directory or a file. when I run the program with "bash lab5", I get "unary operator expected on line 3 and 5" What is the problem? Thanks!

#!/bin/bash

if test "$1" -d
then
    echo "This is a directory"
elif test "$1" -f
then 
    echo "This is a file"
else
    echo "This is neither a file or a directory"

fi