small fix in builder cache key

app/builders/builder.rb

@@ -3,7 +3,9 @@ class Builder
   protected
 
   def self.cache_key(rid, last_modified)
+    last_modified = Time.at(0).to_s if last_modified.nil?
+
     rid + '/' + last_modified
   end
 
-end
+end
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