Cannot have multiple items selected in a DropDownList Error

Cannot have multiple items selected in a DropDownList.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Cannot have multiple items selected in a DropDownList.

Source Error:


Line 13:     function OnSelectedIndexChangecity() {
Line 14:        
Line 15:         var parm1 = document.getElementById("<%=ddlci.ClientID%>");
Line 16:             var strCity = parm1.options[parm1.selectedIndex].text;
Line 17:             if (strCity == "Other") {