--- /usr/lib/python2.6/site-packages/Bio/PDB/DSSP.py.orig	2009-07-15 10:41:45.000000000 -0700
+++ /usr/lib/python2.6/site-packages/Bio/PDB/DSSP.py	2009-07-15 10:39:09.000000000 -0700
@@ -172,6 +172,10 @@
         for key in dssp_keys:
             chain_id, res_id=key
             chain=model[chain_id]
+            # Unnatural amino acids have hetflag != ' ' so the resid
+            # doesn't work as expected
+            if res_id not in chain:
+                continue
             res=chain[res_id]
             aa, ss, acc=dssp_dict[key]
             res.xtra["SS_DSSP"]=ss