PHP - Mysql_error

can someone enlighten me as to what is wrong with this snippet
Code: [Select]
$message_sql = "select * from messages where to_id=$id and read='no'";
$message_query = mysql_query($message_sql);
$message_num_rows = mysql_num_rows($message_query);

i get this error
Code: [Select]
PHP Warning:  mysql_num_rows() expects parameter 1 to be resource, boolean given
what its basically telling me is that the "read" filed doesnt exist..because when I take it out of my query, the query works fine...however I do have a "read" field in my db with  value of "no" stored in it..so im not sure why its telling me that it doesnt exist..