prevent it from trying to downcase a nil element and failing.

systems/quote.rb

@@ -24,12 +24,13 @@ module QuoteSystem
   end
 
   command :quote do |event, *message|
-    category = message[0].downcase
+    category = message[0]
     if category == nil
       quotes = db.execute("SELECT * FROM quotes")
       quote = quotes.sample
       event.channel.send_message("##{quote[0]} <#{quote[2]}>: #{quote[3]}")
     else
+      category = category.downcase
       quotes = db.execute("SELECT * FROM quotes WHERE category = ?", [category])
       if quotes == []
         event.channel.send_message("Uh, this is awkward. The category you're trying to get a quote from doesn't exist.")