Using Multiple Value in Report Parameters

[jpower69]

I'm having a problem using the multi-value check box in the report layout parameters section. I have the following setup
for the user to select multi-semesters for a report..
properties: Nmae: semester, data type = string, Prompt = semester;multi-value box is checked;Available values
from query get semester; value field:semesterID; Label field:description.

This is the code that is getting executed:
if (charindex('-2',@semester) = 0 ) and (charindex('-3',@semester) = 0)
begin
-- Select Chosen Semester
set @wheresemester = ' And semesterID IN ( ' + @semester +')'
end
Problem is that the code works if I select ALL semesters or
if i slect semester = FA2008; it does not work if i selected semester FA2008,SP2008.
I get the following error"CONVERSION FAIL WHEN CONVERTING THE VARCHAR VALUE 4,7 TO DATA TYPE INT"

Can anyone tell me what I am missing here....it is probably staring right at me but i can't see it..

thanks in advance for any information