diff --git a/mmtbx/monomer_library/tst_server.py b/mmtbx/monomer_library/tst_server.py
index 519d203..91ba783 100644
--- cctbx_sources/mmtbx/monomer_library/tst_server.py
+++ cctbx_sources/mmtbx/monomer_library/tst_server.py
@@ -86,8 +86,8 @@ def exercise():
     for base_code in ["A", "C", "G"]:
       rna_atoms = srv.get_comp_comp_id_direct(base_code+"r").atom_dict()
       dna_atoms = srv.get_comp_comp_id_direct(base_code+"d").atom_dict()
-      for as,bs,c in [(rna_atoms,dna_atoms,"d"), (dna_atoms,rna_atoms,"r")]:
-        for a in as.keys():
+      for as_,bs,c in [(rna_atoms,dna_atoms,"d"), (dna_atoms,rna_atoms,"r")]:
+        for a in as_.keys():
           b = bs.get(a, None)
           if (b is None):
             print "Not in %s: %s" % (base_code+c, a)