--- lib/bio/db/pdb/chain.rb	2007-12-18 13:48:42.000000000 +0000
+++ lib/bio/db/pdb/chain_mod.rb	2008-04-01 02:33:44.000000000 +0100
@@ -189,7 +189,7 @@
               x.times { string << 'X' }
             end
             tlc = residue.resName.capitalize
-            olc = (Bio::AminoAcid.three2one(tlc) or 'X')
+            olc = (Bio::AminoAcid.three2one(tlc) or 'X' rescue 'X')
             string << olc
           end
           @aaseq = Bio::Sequence::AA.new(string)
