ClientScript.RegisterClientScriptBlock alerts gives Microsoft JScript runtime error

After ClientScript.RegisterClientScriptBlock  execution it gets  exception as 'Microsoft JScript runtime error: Unable to get value of the property 'options': object is null or undefined'.

 string mortgagorAddress = ds.Tables[0].Rows[0]["BMO_txtMortgagorAddress"].ToString();
            string BMONumber = ds.Tables[0].Rows[0]["BMO_txtBMONumber"].ToString();
            int duplicate = userBL.GetDuplicateInsertion(BMONumber,mortgagorAddress);
            if (duplicate >0)
            {
                ClientScript.RegisterClientScriptBlock(this.GetType(), "Duplicate", "alert('Duplicate Data Exists.');", true);
               return;
      

            }

Above given is my code.