Problem with params and a string

def index
     cond = params[:letter] + "%"
     @clients = Client.find :all, :conditions => ["firstname like ?" ,
cond], :order => "firstname ASC"

This is my code and it gives me an error everytime saying this:

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.+

How can I fix this?

What is the value of params[:letter] ? If it's nil you need to check for that...

if params[:letter].nil? # or possibly .blank?
   # throw a hissy fit