ASSIGNMENT 2 mysql_num_rows() ERROR

Hi Guys

Am having constant issues with an error that I just cant get rid of.
The error is as follows: Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in F:\My Documents\UNISA\ICT2613 PHP\EasyPHP-5.3.3\www\register.php on line 51

Below is a portion the code:
$catch = "SELECT * FROM coffees WHERE username = $userName";
$result = mysql_query($catch);
$num_rows = mysql_num_rows($result); //This row is line 51.

if ($num_rows > 0)
{
$errorMessage = "Username already taken";
print $errorMessage;
}
else
{
$insert = "INSERT INTO user (username, password) VALUES ('$userName', '$password1'";
$result = mysql_query($insert);
}

I've tried everything but it just wont go away and it's affecting the outcome of the program.

Any suggestions would be most welcome

USE FORMATTED CODE TO PSOT CODE AS EXPALINED IN THE TUT LETTER! Do that and you may just receive an answer.