in asp.net about query string

Question

hai,
    i use this code to pass two values in a query string,
while(rdr.read()){
Response.Write("<tr><td><a href=seven.aspx?title1="+str+"id1="+tid+">" + str+"</a>");
}
when i click the title,the title and the appropriate id is also passed as a query string here the id is passed as a null value why,
pls any one can tell me,
Advance thanks,