If parameter isn't OUTPUT, don't pass by reference

sCommand = "exec ap_GetPartIdByPartNo ?sPartNo, ?@iPartId OUTPUT "

>I can't seem to get this to work. SQL is returning Syntax Error:
>
>

>SELECT Import
>SCAN
>
>	sPartNo = ALLTRIM(Import.PartNo)
>	iPartId = 0
>	
>	sCommand = "exec ap_GetPartIdByPartNo ?@sPartNo, ?@iPartId OUTPUT "
>	iResult = SQLEXEC(iConnHandle , sCommand)
>
>	IF iResult < 0
>		AERROR(ArrayName)
>	ENDIF
>
>ENDSCAN
>