I add sound and then it error

i add this script in main function

handle = media_loop("1-22 Central Hall.mp3",null,100);

and when i run the game it said

<handle=media_loop(@4,null,100)>
Test.wdl 53:0 Error(63):Parameter unknown handle

error in script

How to fix it

write `var handle;´ above the function main