diff --git a/rmgpy/data/thermo.py b/rmgpy/data/thermo.py
index 68f3426a65..cfacafd250 100644
--- a/rmgpy/data/thermo.py
+++ b/rmgpy/data/thermo.py
@@ -1738,13 +1738,23 @@ def __addGroupThermoData(self, thermoData, database, molecule, atom):
         if node is None:
             raise DatabaseError('Unable to determine thermo parameters for {0}: no data for node {1} or any of its ancestors.'.format(molecule, node0) )
 
-        data = node.data; comment = node.label
-        while isinstance(data, basestring) and data is not None:
-            for entry in database.entries.values():
+        data = node.data
+        comment = node.label
+        loop_count = 0
+        while isinstance(data, basestring):
+            loop_count += 1
+            if loop_count > 100:
+                raise DatabaseError("Maximum iterations reached while following thermo group data pointers. A circular"
+                                    " reference may exist. Last node was {0} pointing to group called {1} in "
+                                    "database {2}".format(node.label, data, database.label))
+
+            for entry in database.entries.itervalues():
                 if entry.label == data:
                     data = entry.data
                     comment = entry.label
                     break
+            else:
+                raise DatabaseError("Node {0} points to a non-existant group called {1} in database: {2}".format(node.label, data, database.label))
         data.comment = '{0}({1})'.format(database.label, comment)
 
         # This code prints the hierarchy of the found node; useful for debugging