diff --git a/app/builders/builder.rb b/app/builders/builder.rb
index 00973be93271e30449ed922505f78ba80a388da8..2235e8b8dfc591a1f91f9eff0c9831c67b57fb50 100644
--- a/app/builders/builder.rb
+++ b/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
\ No newline at end of file
+end